TALES OF METAL RESILIENCE ROBERT A. FRANCIS Resilience is a popular buzzword today, used in fields as diverse as politics, psychology and ecology to describe the ability to resist adversity. But its original use was in engineering to describe resistance of a material to deformation from applied stress. So, its use to describe resistance to corrosion is not a big reach. Those of us in the corrosion industry are aware of the damage caused by corrosion. But we also know it is possible to mitigate. This series of short items attempts to paint a positive and optimistic picture by reviewing some interesting and historic examples of how corrosion can and has been defeated. Chapter 1 2 3 4 5 6 7 8 9 10 11 12 Title Element 77 - Complete Corrosion Resistance? Space - Free from Rust? Tutankhamun’s Iron Treasure Roman Nails Resist Rust Aircraft Preservation in the Arizona Desert Delhi’s Rust-free Iron Pillar A Stainless Steel Architectural Marvel The Unsung Building Material Zinc Silicate: Liquid Rock for Years of Protection Ever-present Epoxy Coatings Boilers, Steam and Water Treatment The Birth of Corrosion Science Page 1 3 5 7 9 11 13 15 18 20 22 25 Copyright © 2024 Early versions of these articles first appeared in Materials Performance, March 2018 to April 2019. Product names mentioned in this document may be trademarks or registered trademarks of their respective companies and are hereby acknowledged. No copyright infringement is intended. Chapter 1 Element 77 - Complete Corrosion Resistance? Let’s start at the beginning. We know that corrosion is a reaction between a material and its environment, so why not have a look at the most corrosion resistant metal in the periodic table. According to the ASM Metals Handbook, and numerous other sources, that superlative honour goes to one of the rarest elements, Iridium. It is resistant to alkalis as well as highly aggressive acids as shown in the table. The only substances which will dissolve the metal are molten sodium cyanide and potassium cyanide. Corrosive Gold Platinum Concentrated Hydrochloric Acid at 100°C o ✓ Concentrated Nitric Acid at 100°C x ✓ Aqua Regia* at 100°C x x Moist chlorine gas at Room Temperature x o ✓: Resistant o: Some attack x: Severe attack * a mixture of concentrated nitric and hydrochloric acids Iridium ✓ ✓ ✓ ✓ Iridium is a hard, brittle member of the so-called platinum group of metals in the periodic table. It is only a fraction less dense than the densest element, its neighbour osmium in the periodic table and approximately double that of lead. It was discovered by Yorkshire chemist, Smithson Tennant (1761-1813) in 1803, by separating and analysing the residue left after dissolving impure platinum in aqua regia. It was named after Iris, the goddess of the rainbow, because its salts were so colourful. Smithson Tennant World production of iridium is small, only a few tonnes a year, and it is one of the least abundant and most expensive metals. It is produced as a by-product of nickel smelting and has few applications as a pure metal. In such a form, it is used for electrodes in high performance spark plugs and crucibles for the preparation of single crystals of certain electronic and optical glasses. As an alloying element, it increases the corrosion resistance of titanium and palladium and the strength, hardness and corrosion resistance of platinum. Despite its corrosion resistance, you are most likely to come across iridium indirectly. It is one of the mixed metals in Mixed Metal Oxide (MMO) coated titanium impressed current anodes. 1 Those involved in non-destructive testing may come across iridium-192 as an isotope in radiography. There are two interesting facts regarding iridium not related to corrosion properties: • Until recently, the only remaining base unit still defined by an artefact rather than by a fundamental constant of nature was the kilogram, which was defined as the mass of a cylinder of platinum with 10 per cent iridium kept under nested bell jars at the International Bureau of Weights and Measures near Paris. In this case, the iridium is added to increase the hardness to resist wear during cleaning. A new definition of the kilogram based on fundamental physical constants was adopted in 2018. • Iridium is probably best known because of its association with the likely cause of extinction of the dinosaurs. The rocks in the boundary layer between the Cretaceous and Tertiary geological periods, laid down about 65 million years ago, contain a higher proportion of this element than other rocks. An iridium-enriched meteor is believed to have hit the Yucatan peninsula off Mexico, and dust from the impact caused darkening of the skies which wiped out most large animals. Why metals corrode (and some don’t) When a metal corrodes, its atoms lose electrons and the chemical nature completely changes from a solid, shiny substance to charged ions which dissolve in a solution. These ions often react with other compounds to form rust or other corrosion products, but it is this first stage that is critical. Different metals have different tendencies to lose these electrons. The dozens of different metallic elements can be placed in a list which shows their relative ability to lose electrons. A simplified list with some common metals, known as a reactivity or electrochemical series, is shown below. It is very difficult to remove electrons from those at the top, such as gold and platinum. But those at the bottom, such as calcium and sodium are very reactive and rarely found in their native form. In between, metals show an increasing degree of reactivity moving down the list. Chemists call the reaction which removes electrons oxidation. A chemical or chemicals have to react with the metal to take up electrons from the oxidation reaction. As the name suggests, oxygen can take up the electrons and form a metal oxide. There are many other compounds of varying ability that can oxidise metals. These are shown in the right-hand column. Water will oxidise metals at the bottom of the reactivity series, while acids are necessary to corrode those in the middle. Very strong oxidisers are required to attack the stable metals at the top. The series can explain the relative resilience of many of the metals, but there are some anomalies. Aluminium, chromium (which gives stainless steel its resistance) and, especially, titanium are known to be much more resistant than their position in the series would suggest. These metals form a protective oxide film in many environments so can resist many oxidisers. But this oxide film, known as passive protection, can break down under certain conditions, such as a high concentration of chloride ions. Metals arriving from outer space bring us to our next tale in success in resisting corrosion. 2 Chapter 2 Space - Free from Rust? Corrosion, being a reaction between a material and its environment, can be minimised by creating a benign environment, rather than looking at expensive materials such as iridium. And, in theory, it is not hard to find a very benign environment – simply head upwards for one hundred kilometres or so into space. No water, no oxygen and no chemicals to cause corrosion. Of course, there will be all sorts of other trepidations such as temperature and pressure extremes, fast moving space dust and rocks, high UV and radiation exposure, any of which may destroy many materials. But true chemical corrosion is unlikely to be a concern. If, or when, a base is set up on the moon, inhabitants will need to protect their materials from the natural environment. The high UV radiation will damage many organic materials such as paints and plastics. The sharp moon dust, which is not subject to wear and rounding from weathering we see on earth, will destroy exposed bearings and any other contacting surfaces. The Moon exploration as imagined in an early 1960s children’s book. extremes of temperature will cause thermal fatigue of metallic and non-metallic materials. The absence of corrosion from rain, oxygen and marine salt is unlikely to be much comfort to engineers designing human habitation and facilities on the moon. The spacesuit worn by Neil Armstrong on the moon has been found to be degrading and currently undergoing restoration. But it is neither the space nor the moon environment that is the culprit. Plasticisers have from leached from the PVC, rubber has broken down and degraded, physical damage has arisen due to poor handling and storage and sweat from the astronaut activity have all combined to destroy plastic and metal components. It is hoped that conservation efforts will significantly slow such damage, but it does seem a shame that one of the most important symbols of our age may not be seen by our descendants. Our sister planet Venus may also seem to be a corrosion nightmare. A number of Russian Venera probes landed on the surface in the 1970s and 1980s, but only sent back data for an hour or so. Venus does have corrosive sulfuric acid in its upper atmosphere, but at the 3 surface it is the high temperature (about 450°C) and high pressure (about 90 times that on earth) that would have destroyed these landers. The titanium spacecraft would probably now be twisted bits of metal on the surface, but corrosive metal loss would probably be minor. Away from physical problems of heat and pressure, space is remarkably benign. The building blocks of our solar system are rocky and metallic asteroids which have been circling the sun for billions of years. Metallic asteroids are largely made of iron, with about 10 per cent nickel and traces of other elements (such as iridium in the meteor that caused destruction of the dinosaurs) and only have a surface tarnish from the solar wind after all those years. The picture shows a slice of the Muonionalusta meteorite found in Sweden in 1906. (The etch, known as a Widmanstädten pattern, arises from the very slow cooling of the metal during its formation). Very little surface oxidation has taken place as it was buried deep after it fell to earth about a million years ago. Such an alloy would readily corrode on earth and meteorites found on the surface oxidise when exposed to water and oxygen. But in space they remain largely in the state they were when they formed at the birth of the solar system. We return to earth but stay with meteoric iron for our next corrosion-free artefact and see an example of how iron can remain rust-free for thousands of years. 4 Chapter 3 Tutankhamun’s Iron Treasure When he discovered the tomb of Tutankhamun in 1922, Howard Carter was asked what he could see. “Wonderful things!” he replied as he peered into the treasure-filled tomb. Of the many “wonderful things” Howard Carter found, a small iron dagger may seem insignificant compared to the numerous gold, silver and beautifully-coloured objects that the boy king is famous for. But to the corrosionist, this item is remarkable because it just may be the oldest intact iron artefact known. The iron dagger, about 34 centimetres long with a gold handle, was found in the sarcophagus of the ancient Egyptian king, who was buried in 1320 BC. Researchers from the University of Pisa and other establishments in Italy and the Egyptian Museum  recently analysed the blade using non-destructive x-ray fluorescence and found that the composition was of meteoritic iron with about 10 per cent nickel and 0.6 per cent cobalt. Ancient peoples used meteorite iron for various tools, weapons and jewellery, although this dagger shows a high degree of metalworking skill and is probably the finest example of its type. Despite being buried for nearly 3,300 years, the blade is in excellent condition, with only minor evidence of corrosion as shown in the photograph. The inset shows the surface is not completely smooth or clean, but no analysis of the surface contamination has been reported. There are a number of possible causes for this surface stain. Iron meteorites are not completely metallic, and there may have been non-metallic silicate or other inclusions. The method of iron working is unknown, but heating and forging may have embedded charcoal or other impurities in the surface. Finally, the dagger was actually in the sarcophagus next to the mummified body. Mummification involved packing the body with a rock salt material, natron, to desiccate the body and this corrosive material along with bodily fluids may have caused some initial corrosion before completely drying took place. Any or all of these may be the culprit for the surface defects. While 10 per cent or so of nickel does reduce the corrosion rate of iron somewhat compared to mild steel, it would still be expected to behave similarly to modern steels. If exposed to the atmosphere or soil, even in an environment as arid as that in Egypt, it would Daniela Comelli et al, “The meteoritic origin of Tutankhamun’s iron dagger blade”, Meteoritics & Planetary Science 51, Nr 7, 1301–1309 (2016)  5 rust and little if any of the original metal would be left. An ancient meteoric iron dagger found at Alacahöyük in Turkey, admittedly a thousand years older than that found in Tutankhamun’s tomb, no longer contains any metallic iron. X-ray investigations show this artefact has completely rusted away, although the gold hilt is still intact. It is not the nickel in the iron that has allowed Tutankhamun’s dagger to remain intact but rather the very dry conditions in the tomb. The Valley of The Kings where pharaohs were buried is a very dry environment. as the picture shows. Egyptian tombs were completely sealed once the complex embalming and funeral process was completed. Any moisture would quickly react with wood or other contents on the tomb. The air would remain dry while the tomb remains undisturbed, preventing not only damage to the iron dagger, but to wood and other organic items. Sadly, the humidity from the many tourists that have passed through the tomb since it opened has caused the remaining items and wall murals to become damaged. The Egyptian Museum occasionally sends some of Tutankhamun’s treasures to overseas museums for special displays, and these treasures include a gold dagger. The iron dagger stays in Cairo – it is too valuable. At around the time of King Tutankhamun, ancient Anatolian peoples in what is now eastern Turkey were making what is one of the most important discoveries in technological history. The Hittites discovered the difficult process of how to smelt iron from its ore and the Iron Age was born. Again, most of the iron produced has since rusted away but there are examples of ancient man-made iron still extant which further show us it is possible to defeat corrosion. We will look at one of these instances in the next chapter. 6 Chapter 4 Roman Nails Resist Rust In about 88 A.D., a remote Roman legion based at a fortress at Inchtuthil on the north bank of the Tay River, not far from what is today Perth in Scotland, was ordered back to the European mainland. They dismantled or burnt the buildings, smashed the pottery and filled in drains and sewers to prevent them getting into the hands of the local Caledonian tribes. Unable or unwilling to transport nearly a million iron nails south with them, they dug a pit and buried them. Iron could be used for weapons, agriculture and buildings in those days, so highly prized by the enemy of the Roman legions. The Romans did a good job of hiding them, because it was nearly 1900 years before they were found. In 1961, an archaeologist, Professor Sir Ian Richmond, noticed a region of different coloured earth at the Inchtuthil site and, after digging down some metres, came across a large corroded mass of iron consisting of nearly 10 tonnes of nails and other small iron implements. The picture shows part of the haul. Further investigation found that, while the outer items had badly corroded and formed a solid crust of iron oxide, those on the inside had only a minimal amount of rust on the surface. The survival of the inner nails was attributed to the protection from corrosive water and oxygen achieved by barrier created by the outer solid crust. It is not possible to know how little water or oxygen the inner nails were exposed to over the centuries, but clearly the amount was very small, or the time it was exposed to these was very short. The investigation carried out at the site was archaeological, not a corrosion investigation and no information on water or soil composition or other site chemical factors was obtained. However, survival of iron on a river flood plain for nearly two millennia gives further evidence that protecting iron from water or oxygen exposure or both can prevent or at least minimise corrosion damage. Many of the nails were sent to museums, corrosion associations and other interested persons around the world as gifts. Nails of different sizes were mounted in a wooded box with a clear, plastic lid along with a white plastic label reading “Iron nails from Roman legionary fortress at Inchtuthil, Perthshire, Scotland AD 83 - 87”. The photograph shows the box sent to the Australasian Corrosion Association (ACA). This contains five nails of square shank section with flat, circular heads, of approximate lengths 19.5, 17, 13.5, 9 and 5 centimetres. The nails are now in rather poor condition showing dark flaking rust, especially 7 on the larger nails. Similar boxes appear occasionally on auction and other web sites, and most of the nails on offer also appear to be severely corroded. Photographs of some nails in their original condition show far less corrosion. It may be that exposure to humidity over the past fifty years has caused the small amount of rust that was on them originally to grow. However, it is more likely that acetic acid fumes from the wooden box has been the main contributor to the additional corrosion. The fact that corrosion products show black magnetite, typical of rust formed in severe environments, would support this hypothesis. Metallurgical investigations of such nails have been carried out since their discovery and show the microstructure and properties typical of ancient wrought iron objects. Iron could not be obtained as a liquid from iron ore at temperatures and technology available to the ancients, but the Hittites who lived in modern day eastern Turkey are believed to have discovered in about 1300 BC that heating iron ore to around 1200 degrees Celsius in a charcoal-fuelled furnace reduced the iron oxide to iron metal and hammering enabled the impurities to be driven out as slag. The result was a strong and tough metal consisting, as the micrograph shows, of a mixture of almost pure iron with the remaining slag forming longitudinal inclusions (known as stringers) spread throughout the metal. In regions where the metal contacted the charcoal in the furnace a higher carbon content was achieved. The technology spread across the ancient world over subsequent centuries west to Europe. As with Tutankhamun’s dagger, the Roman nails show that it is possible to protect a metal from corrosion by minimising exposure to moisture. For our final lesson in the importance of keeping metal dry, we return to the present and visit a US Air Force base in the Arizona desert where the low humidity has enabled the military to minimise corrosion damage to its old, unwanted and obsolete aircraft. 8 Chapter 5 Aircraft Preservation in the Arizona Desert The Davis-Monthan Air Force base in Tucson, Arizona, USA, commonly known as ‘The Boneyard’, is probably the strangest air field in the world. It is the place where the US Air Force and other federal agencies send their obsolete, unwanted, excess and out-of-service aircraft, many of which will never fly again. With around 4000 aircraft, it is the largest such storage and preservation facility in the world, and would be the world’s second largest air force! Planes have been stored here since the end of the Second World War. Types range from F-15 and F-16 fighters, Hercules and Galaxy transports, A-10 attack planes and B-1 nuclear bombers. The photograph shows C-135 transport aircraft. There are also Harrier jump jets, experimental aircraft, helicopters and missiles. Not all are intact. Many have been cannibalised for spare parts, but there are other reasons for destruction. The tails were sliced off hundreds of B-52s as part of a post-Cold War treaty with Russia to provide proof they had been scrapped. The parts were scattered on the ground so that Russian satellites can confirm their destruction. Although it is commonly perceived as an aircraft graveyard, a sizable percentage will fly again. About ten percent are kept in good condition in case they will need deployment in the future by the US or its allies. The Australian air force, for example, sourced a batch of second-hand F-111 bombers, an example shown left, in 1992 in a bargain-basement deal with the Pentagon. Some are used as target drones. Many F-4 9 Phantoms, one of the most important aircraft of the post-war period, have ended their lives in such an ignominious manner. Its chief mission, however, is supplying the air force with spare parts. Parts from the Boneyard can make the difference between an aircraft being grounded or operational. Only if it is not required for any reason, and has given all the spare parts it can, is it scrapped. Many have valuable metals such as gold and platinum. When an aircraft arrives at the facility, weapons and classified hardware are removed. They are then thoroughly washed, which is especially important for marine aircraft or those employed in tropical locations. Fuel is flushed and mechanical parts are protected with a lightweight oil. Canopies are sealed with a white vinyl latex coating (Spraylat) to protect cockpit interior from heat and stop the canopies becoming crazed and cracked. Joints are sealed from dust. Once this is done, the aircraft is towed to its storage area. The site Tucson provides an ideal site for such a facility. The hard soil is strong enough to take the weight of a 300 tonne aircraft and makes it easy to move aircraft around without the need for pavement. However, it is the climate and clean air that minimise degradation that are most important. It has a very low average humidity of 38%, meaning moisture contact (time-of-wetness) is minimal. It is hundreds of kilometres from corrosive sea salt, the hot climate mean de-icing salt is not required to melt snow, and there is no pollution from heavy industry. As a result, the corrosion rates would be very low. Phoenix AZ, with a similar environment, has a carbon steel corrosion rate of approximately 5 microns per year, putting it near the benign end of ISO C2 Low Corrosivity category. A Note on Humidity and Corrosion It should be stressed that the low average humidity figure does not tell the whole story. Like temperature, relative humidity fluctuates during the day and night and over the year. The chart shows historical morning (close to average) and afternoon (close to the minimum) readings throughout the year (solid lines) for Tucson, Arizona. However, the maximum humidly is normally reached at sunrise; well before meteorologists arrived at work (Historically, such data were recorded manually before electronic instruments and data recorders became available). The maximum humidity can be estimated (dotted line) as following similar behaviour to average and minimum humidity. This shows that quite high humidity values can be reached. A still, clear summer night could easily decrease metal temperatures to well below the dew point, resulting in condensation over the exposed surfaces leading to corrosion. The low average relative humidity should not be construed as indicating that corrosive conditions will not arise. For our next example of resilience of iron against the elements, we will see how ancient Indian smiths accidently found that impurities in the iron can completely change its corrosion behaviour and created a famous rust-free icon. 10 Chapter 6 Delhi’s Rust-free Iron Pillar In the courtyard of a ruined mosque at the Qutub Minar complex not far from Delhi’s international airport in north India, stands a pillar of iron that has remained rust-free since it was fabricated in approximately 400 AD. The six tonne pillar was originally forged and erected in central India during the Gupta period, but moved to its present site about seven centuries ago. Although the climate in Delhi is not particularly corrosive at about 20 microns per year for mild steel (ISO category C2), this cannot explain the rust-free condition of the pillar. As a result of its remarkable state, it has fascinated historians and archaeologists, as well as those of us with scientific and technical interests, for many years. The solid iron pillar is about 7 metres high and about 40 centimetres in diameter at the base. The lower part is rough and pitted where it was once below ground but the rest of the cylindrical column is smooth and tapers to a decorative bell at the top. An inscription commemorates the victory of King Chandra over his enemies in the fourth century AD. There is also damage believe to be from a cannon shot. The massive pillar was not built in one piece, but rather by hammering together several pieces of hot wrought iron in a process known as “forge welding”. The composition is not homogeneous, with the carbon content especially varying widely as with other ancient wrought iron. The average composition is shown in the table, compared to a typical modern steel of similar carbon content. 11 Carbon Manganese Silicon Sulphur Phosphorus Delhi Pillar 0.15% 0.05% 0.05% 0.005% 0.25% Typical modern mild steel (1015 grade) 0.15% 1.2% 0.3% 0.01% max 0.02% max The level of impurities is generally low, typical of ancient wrought iron, apart from the phosphorus level which is over ten times that of a modern steel. A high phosphorus iron ore was clearly used and this element is present in the iron. As a result, a very thin dark grey protective layer of crystalline iron hydrogen phosphate has formed on the surface of the pillar. Subsequent amorphous (noncrystalline) rust forms which is compact, dense and adherent with low porosity which is the reason for its resistance to corrosion. Any crystalline rust that forms is porous, easily flakes off and does not contribute to protection. Surely then we just need to increase the phosphorus content of steel to prevent it rusting? Unfortunately, this amount of phosphorus has a detrimental effect on the mechanical properties. While the Delhi pillar iron has quite a high yield strength typical of modern structural steel, the phosphorus has greatly reduced the ductility as measured by elongation. The impact toughness would be similarly reduced. A steel with such a high phosphorus content would be far too brittle and suffer a problem that used to be known as ‘cold shortness’ (‘short’ is old term meaning brittle, as in short bread). It was recognised many years ago that there needed to be limits on phosphorus in steel. In fact, the first standard issued by ASTM in 1901, ASTM A1 for railway rails, included a limit on the phosphorus content. A high phosphorus content doesn’t matter for a pillar that just needs to sit there, but is totally unacceptable for anything that is going to be subject to stresses, such as a bridge, building, rail or any mechanical device. Yield strength Tensile Strength Elongation Delhi Pillar 324 MPa 330 MPa 5% Typical 350 MPa steel 345 MPa 450 MPa 25% So, the ancient Indian blacksmiths (accidently) achieved rust-free iron 1600 years ago, but unfortunately at the expense of useful mechanical properties. In the next instalment, we will see how clever manipulation of the chemical composition can produce truly rust-free steel as well as acceptable mechanical properties and has been used to produce one of the modern wonders of architecture.  Matthew V. Veazey, “Materials Performance” 44(7) p16, July 2005 12 Chapter 7 A Stainless Steel Architectural Marvel With the Delhi Pillar, improved corrosion resistance was achieved through alloy additions to the iron, but this was the result of luck with the ore used. It was not until the late Victorian era that scientific investigations produced the precursors to the modern corrosion-resistant steels in use today. The discovery in 1882 of Hadfield’s manganese steel with its remarkable wear-resisting properties probably started serious investigations into means of making major improvements in steel properties by controlled additions of alloys. However, attempts at improving the corrosion resistance of steel had commenced early in the nineteenth century. Michael Faraday carried out experiments alloying iron with noble metals such as platinum, although the results were disappointing. Harry Brearley in Sheffield is often considered to be the inventor of stainless steel, but in fact there were important discoveries made before he prepared the first martensitic stainless in 1913. In Harry Brearly Philip Monnartz Eduard Mauer 1911, Philip Monnartz in Germany discovered three important facts regarding stainless steel. Firstly, the correlation between chromium content and corrosion resistance; secondly, that a significant boost in corrosion resistance is achieved when the chromium content exceeds 10.5 per cent and finally that molybdenum can have an importance influence on corrosion resistance. Monnartz is also the first to use the term ‘passivity’ to describe the improvement in corrosion resistance. Concurrently, Eduard Maurer and Benno Strauss at the Krupp works in Germany alloyed steel with both chromium and nickel to produce the first austenitic stainless steels. Others such as Elwood Haynes in America also claimed to have invented a corrosion-resistant steel at around the same time. However, it was Brearley who recognised the commercial significance of stainless steel and developed the large scale production of the metal, initially for cutlery. In the early 1920s, a whole variety of chromium and nickel combinations were tried with the best known being the18/8 or 304 stainless steel (18 per cent chromium, 8 per cent nickel). The addition of nickel produced a stainless steel that was more ductile, and more resistant to acid. Most of the standard grades produced in these early years are still in use today. 13 The first major use of stainless steel in architecture, an example that arguably has not been surpassed, is the Chrysler Building, erected in New York City in 1930. Walter Chrysler engaged architect William Van Alen to design the tallest building in the world, and at the same time produce a building that would promote his automobiles. A Krupp version of 18/8 stainless known as ‘Nirosta’ was selected as the cladding metal. It had gargoyles which were supposed to represent Chrysler hood ornaments. The stainless-clad radiating arched crown in a sunburst pattern is the most impressive feature of the Art Deco masterpiece. When the spire was added in 1930, it was the tallest building in the world, but less than a year later it was overtaken by the nearby Empire State Building. The building was declared a National Historic Landmark in 1976, and is often first in polls of architects, engineers, historians and builders as their favourite building in New York. A committee was set up to inspect the condition of the building every five years as there was an obvious interest in the durability of stainless steel in architectural applications. This committee was disbanded in 1960 because there had been virtually no deterioration of the panels. The top of the building has been cleaned manually only twice, in 1961 and 1995. During the 1995 cleaning, some dents and cracks were observed and repaired, and superficial pitting near an incinerator vent was detected. But the condition of the stainless was found to be excellent, looking like it had just been installed. As one commentator has noted “With its stainless steel crown gleaming in the sun … the Chrysler building always looks like the future.” Of course, stainless steel is widely used in many applications other than architecture. PBS reported in its television programme “The Streamliners”: “Stainless steel, with its sleek, shiny surface and tremendous strength, is a marvel of technology. It has revolutionized most modern industries, including food, medicine, and transportation. … Stainless steel paved the way for modern technology and continues to influence our lives every day.” Alloying to make steel corrosion resistant was effective, but most applications required something much cheaper. Therefore, putting a barrier between the steel and its environment, a protective coating, has become the single most important means for preventing corrosion. And, as we shall see in the next chapter, the most effective way is to apply a thin layer of zinc to the steel. Further reading: Harold M Cobb, “The History of Stainless Steel”, ASM International, (2010). 14 Chapter 8 The Unsung Building Material Corrugated iron is a widely-used building material whose importance is rarely acknowledged. It is used in all climates, all terrains, cheap, easily fabricated into all sorts of buildings and can probably claim to shelter more people from the elements than any other building material. Corrugated iron, often incorrectly called ‘tin’, is usually coated with zinc so terms such as ‘corrugated galvanized iron, ‘galvanized iron’ or even ‘galvo’ are often used. But like all great inventions, it has evolved over time so we now have coatings such as zinc-aluminium, zinc-aluminium-magnesium along with special durable organic coatings providing a wide range of colours. The durability of the new products has improved so much that it is the fasteners, not the sheeting, that often break down first. But regardless of the coating, this simple material made of a thin sheet of steel, crinkled for strength and rigidity, then provided with a suitable protective coating, has contributed to development of farming, light industry, housing and many other fields of human endeavour like no other building material. Today, corrugated metal sheets are made by a roll-forming process where wavy rolls press the sheets to give the desired corrugations. The sheets are then cut to the desired length. Rolling and coating is now a completely mechanised process. The corrugations provide greatly increased stiffness and rigidity compared to flat sheets along their length, but also give the ability to be shaped into curved forms. The pitch or distance between corrugations determines the strength, with lowest pitch (‘ripple iron’) having the lowest strength and generally only used for internal applications. A number of new profiles have been introduced in recent years providing a range or architectural options. 15 The first patent for ‘indented or corrugated metallic sheets’ was granted to Henry Palmer of the London Dock Company in 1829. The company purchased flat wrought iron sheet, rolled it to shape using fluted rollers and a year later the first building roofed with this material was constructed at the London docks. The roofing was an immediate success and railway stations, markets and other large span enclosures in England were soon roofed. These first examples were protected with a thin layer of paint, which provided limited protection from corrosive smoke, steam and salt air. The problem was addressed by Stanislaus Sorrel, a French civil engineer, who in 1837 patented a process of coating iron with a thin coat of zinc, although the sheeting was still often painted for decorative purposes. By the 1850s, mass production and increased competition led to more widespread use of the material. The gold rushes in California and Australia in the 1850s led to a market in prefabricated buildings that would be light weight and quick and easy to erect and corrugated iron formed the basis of the first flat-pack industry. Even after the gold rushes subsided, corrugated iron remained an ideal building material in remote areas and by the outbreak of the First World War, it was a common feature of rural landscapes around the world. But it was also widely used in built-up regions in domestic, commercial and industrial applications. By this time, steel had replaced wrought iron as the base material and it had become cheaper. War brought perhaps the most famous corrugated iron building, the semi-circular Nissen hut and its American version, the Quonset hut, where the lightness, ease of transport and erection made them a great success with the military forces. In recent years, the product has managed to shake off its rather utilitarian image through the work of a number of imaginative architects who have used it to produce buildings of elegance and style which fit in well with the local landscape. If corrugated iron has been important in the developed West, then it is almost a currency in the developing world. Since the Second World War, informal communities with populations of a few hundred to a million or so have sprung up on the periphery of urban centres in Africa, Latin America and south Asia. Without land title or money, the residents have adopted corrugated metal as a flexible, cheap and easy to work with building material. It is suited to hot and temperate 16 climates; it can withstand monsoon rains and is rot-proof. It can be easily erected and pulled down and when no longer needed for housing, it can be used for channelling water, covering muddy paths or an animal enclosure. As David Egerton has remarked: “Cheap, light and portable, [corrugated iron] is one of the great technologies of poverty, together with the T-shirt and the flip-flop.” How Zinc Protects Steel Coating steel with zinc is perhaps the most important means of protecting steel. It will provide barrier protection while intact as its corrosion rate is much less than that of steel. But more importantly, if there is a pinhole or damage to the zinc coating, then the zinc corrodes in preference to the steel providing sacrificial protection, and we say that the steel is galvanically protected by the zinc. The zinc can be applied by a number of methods apart from dipping in molten zinc, such as electroplating and thermal spray. Zinc powder can be dispersed in a paint binder to form a zinc-rich paint. The protection given by the zinc depends on the coating thickness, with greater protection given by thicker coatings. The environment is also important, and coating life diminishes as the environment becomes more corrosive. Corrugated iron has been and continues to be an invaluable building material that deserves to be taken seriously. It shows that steel can be simply and cheaply formed to a useful shape, protected from corrosion by a range of coatings, and made accessible to citizens in all corners of the globe. But for heavy steel sections in severe environments, other coatings such as inorganic zinc silicate, discussed in the next chapter, are needed. Further reading: Adam Mornement & Simon Holloway, “Corrugated Iron”, Frances Lincoln Ltd, (2007).  David Egerton, “The Shock of the Old; Technology and global history since 1900” (Profile Books 2006). 17 Chapter 9 Zinc Silicate: Liquid Rock for Years of Protection At the outbreak of the Second World War, the Australian government decided to build a blast furnace and shipyard near the then main source of iron ore at Whyalla in South Australia. However, as this site was on the edge of a desert, fresh water needed to be piped 360 kilometres from Morgan on the Murray River. An above-ground welded steel pipe was proposed and the local Water Authority, responsible for the project, considered possible coatings. After investigation of possibilities available at that time, they decided that ‘nothing could hold a candle’ to the newly developed inorganic zinc silicate. At this time, most steel was protected with oil-based coatings which needed re-application every few years. Their faith was vindicated as the original coating, shown in the photograph near the Whyalla steelworks, is still largely intact over 70 years later. In fact, inorganic zinc silicate either alone or part of a coating system, now probably protects more steel from the ravages of corrosion than any other single protective coating apart from galvanizing. Inorganic zinc was developed by an Australian inventor working in his shed in suburban Melbourne. Victor Nightingall (1881-1947) had many interests including agriculture, electric heating and x-rays and had taken out a number of patents. In the 1930s he decided to try to develop a totally new type of protective coating by reacting sodium silicate (like a liquid glass) with zinc powder and applying it to a steel surface. He was hoping that the components would react together to form an iron-zinc-silicate mineral rather like a mixture of the minerals Williamite and Franklinite. We know now that the extent of reaction was 18 restricted to the surface of the iron and zinc particles, but after much experimentation he came up with a successful coating that clearly provided superior protection than any other product on the market. He coined the term ‘dimetallization’ to describe the reaction between the zinc and iron, and formed the Dimet Company to market the product. The coating required heating to cure it to sufficient hardness, but it was used successfully on a small number of projects in Melbourne before being adopted for the Morgan-Whyalla pipeline. After the war, it was adopted for other similar pipeline projects around Australia and became widely used for other applications where the stoving process was not an issue. In 1949, Charles ‘Chuck’ Munger of the Ameron Company visited Australia to inspect the Morgan-Whyalla coating, and other similar projects. He was impressed and licensed the product for use in the United States, where it was marketed as Dimetcote. The stoving process was, of course, a severe limitation and work by Ameron and in Australia developed a post-cured product where a curing agent was washed onto the coating after application. By the 1960’s, self-curing water-borne products were available, soon followed by solventborne products, simplifying application even further. These are the products in use today and are marketed by most large paint companies around the world. Inorganic zinc silicates are widely used for steel structures where the grey colour is acceptable, such as road bridges as shown left. They are remarkable coatings and unlike other paints in many ways. Being inorganic, they are unaffected by sunlight, ultra-violet radiation, rain, dew, bacteria, fungi or temperatures up to about 400°C. If they are damaged or worn away, the zinc continues to provide galvanic protection to the underlying steel and porosity in the coating is filled with corrosion products. As the zinc is further consumed, rust may start forming but will not grow under the intact zinc coating, unlike normal paints. They resist most organic solvents and have excellent friction characteristics. In fact, they are usually the only paint coating allowed when structures contain friction-grip joints. Most importantly, a properly applied single coat of inorganic zinc will give excellent protection to steel for years or indeed decades, even in some quite aggressive environments. They are truly unique and it is little wonder that they have revolutionised the means of protecting steel structures from corrosion. 19 Chapter 10 Ever-present Epoxy Coatings If you work in corrosion control, you will undoubtedly come across epoxy coatings. They are used wherever steel has to be protected against corrosion – offshore platforms, onshore oil and gas production, refineries, petrochemical plants, power plants, bridges, ships, wharfs and many more. They will protect steel in the atmosphere from rusting by preventing oxygen and water reaching the surface. They will prevent the interior of steel tanks from reacting with cold water, hot water, petroleum and other organic products and many other acidic, alkaline and pH neutral chemicals. For good measure, they are also used to protect other surfaces such as concrete and non-ferrous metals. If you had to name the most versatile method of preventing corrosion, an epoxy coating would be a good choice. So, what is it that makes epoxy such a useful chemical? The term epoxy refers to thermosetting polymers (one that cures irreversibly by molecules cross-linking) produced by reaction of an epoxide group (often also confusingly called an epoxy group), which is a three-membered ring of an oxygen atom attached to two adjacent carbon atoms. The ring structure of the epoxide group is easily opened and provides a site for crosslinking to form the final polymer. The epoxide group is on a small molecule, most commonly epichlorohydrin (ECH) which is reacted with a much larger molecule containing lots of carbon ring (aromatic) and chain (aliphatic) structures, most commonly bisphenol-A (BPA) to form what is known as an “epoxy prepolymer”. This is a large polymer molecule, but still a liquid, and forms the resin half of the final product. It needs to react with a hardener (also called a converter) to cure to form the final, highly crosslinked product that is strong, hard and provides the required corrosion resistance. No volatiles are given off during cure so shrinkage is much less than for other polymers. One common hardener is polyamine or, simply amine, which is an organic molecule containing nitrogen and hydrogen atoms rather like ammonia. The hydrogen atoms on the amine react with the oxygen atoms on the epoxy prepolymer to form the three-dimensional cross-linked polymer that gives the final product its excellent properties. The polymer has many charged molecular groups such as hydroxides which make it polar and ensure good adhesion to metallic surfaces. Furthermore, the carbon-carbon and other linkages along the polymer chain are strong and very stable giving epoxies their good chemical resistance, strength and toughness. At the same time, cross-linking is just sufficient enough to give the film some flexibility. There are a range of epoxy types depending on the resin or hardener used making epoxies one of the most versatile polymers. Other resins than BPA include bisphenol F and novolac which have improved resistance to certain chemicals. As well as amine, another common hardener is polyamide, which has lower shrinkage on curing, increases the flexibility of the 20 film and gives a longer pot life, but has reduced chemical resistance. There are different formulations for different applications. For atmospheric coating systems, a polyamide hardener is generally used, and there are pigmented and non-pigmented primers and intermediate coats. The major weakness of epoxies exposed to the atmosphere is that they chalk due to reaction with UV, so they not used as topcoats where colour or gloss are important, although chalking has only a minor effect on durability. For maintenance work where abrasive blasting is often not possible, surface tolerant epoxies (or epoxy mastics) which are formulated to soak into poorly prepared surfaces as well as dry slowly are the usual primer of choice. For more aggressive environments such as splash zones, very high build or ultra-high build epoxies are often specified. For tank linings, solventless epoxies with or without glass reinforcement, are ideal. For pipelines, powder coatings (fusion bonded epoxies) are used alone or as part of a three layer system. Various additives and modifications can create products for high temperatures, wear and abrasion resistance, fire resistance and non-slip flooring. It is not surprising that most corrosion control activities will use epoxy coatings at some stage. Epoxy coatings are widely used for tank floors, piling and reaction vessels The first true commercial bisphenol epoxy resins were discovered in the late 1930s by Pierre Castan (1899 – 1985), pictured, working in Switzerland for the company that eventually became Ciba-Geigy. He was attempting to find a product which could be used for dentures, but this was not commercially successful. At around the same time, Sylvan Greelee working for the Devoe-Raynolds in the USA was reacting bisphenol-A with fatty acids to produce an air drying coating. Various chemical companies developed this original research after World War 2 and commercial epoxy resins for coatings made their debut in the late 1940s. Protective coatings were one of the first and are still the most important application of epoxies accounting for around 50% of all epoxy resins produced, but epoxies are also found in adhesives, castings, flooring and many other uses. 21 Chapter 11 Boilers, Steam and Water Treatment It is difficult for us in the 21st century to imagine some of the hazards and dangers that were associated with the Industrial Revolution as it spread across the developed world two hundred years ago. It was steam that drove the revolution with boilers providing the energy for the locomotives and steam ships, as well as the stationary engines that ran the factories. But the rapid development of steam power brought many disasters. In 1815 in northern England, an experimental railway blew up killing 16 spectators. The boiler in the steampacket “Union” exploded in the English port of Hull in 1837, killing 20 and injuring the same number. But it was in the USA that boiler accidents were most common and tragic. The boiler in a printing press and machine shop in Hague Street, Manhattan exploded in 1850 killing 67 and injuring 50. In April 1865, the boiler in the paddle steamer “Sultana” blew up with an estimated cost of 1500 lives, more than the loss on the Titanic. Railway boilers often suffered catastrophic explosions, as shown in the photograph. Between 1880 and 1890 there were over 2000 boiler explosions in the USA alone. There were many reasons for these and other similar disasters. Boilers were originally made by riveting together small sheets of wrought iron. Its strength was adequate for the relatively low pressures in the early days, but the higher boiler pressures necessary for larger trains, ships and stationary engines required stronger materials which were not available or too expensive. Operating guidelines were unknown, steam pressures were regularly increased beyond a safe level (safety valves were unreliable and frequently screwed down) and inspections were rare. Corrosion, cracking and problems arising from scaling and fouling were not the most important boiler problems, but there is no doubt that 22 the attention given to these from the late 1800s on was an important step in improving boiler safety. Understanding and dealing with the corrosive and scaling properties of water so that boilers could be safely and efficiently operated, was an important technological development that has largely been overlooked. As steam is produced in a boiler, the impurities left in the feed water concentrate and they precipitate out of the solution and onto the internal surfaces of the boiler causing fouling, generally known as incrustation in the 1800s. This reduces both the water flow and the heat transfer decreasing the efficiency and power of the boiler. Such fouling can accelerate corrosion of the boiler, decreasing its life. Impurities can also cause foaming from bubbles which burst as steam is generated also degrading boiler operation. Corrosion was considered less of a problem than fouling as boilers often had a very limited life of five years or so, but became increasingly important as boilers became more complex and scaling problems diminished. For marine steamships, salt water was used in the boiler until the mid1850s, and the incrustation it produced required continual cleaning. Steam locomotives exhausted to the atmosphere so water had to be constantly replenished. Over the course of a route, considerable variations in water quality were found creating different types of incrustation. The problems were less with land boilers as the feed water was constantly recycled. Rather than attempting to treat the water in the first place to prevent fouling arising as we would do these days, the problems were initially dealt with by regular cleaning of the boiler. Removing scale was a slow and arduous process, and numerous means were found to make it easier. These methods were discovered serendipitously or by trial and error. The table from Molesworth’s Pocket Book of Engineering Formulae of 1888 gives some incrustation removal methods that were adopted with more or less success. For example, in what would appear to us the strangest solution, it was found that boiling potatoes in the boiler water (Item 1) made scale removal easier. Apparently, the starch in the potatoes caused a soft sludge rather than a hard scale to form. Tannins and lignin from wood, plant matter and other organic sources described in items 2, 3, 7, 8 and 11 had a similar effect by forming a soft sludge. Tannin had other advantages such as removing oxygen and is still used today in some water treatment systems. It does react with iron, especially under neutral or acidic conditions, and its use needs to be carefully monitored as noted in item 7. Molasses (item 6) also forms a soluble compound with calcium salts, making sludge removal easier. Keeping the water slightly alkaline (items 2, 4 and 9) is still standard practice for water treatment to minimize corrosion risk. Phosphate is another treatment used in the early days which is still 23 in use today. Frequent blowing off (Item 12) to replace contaminated water with fresh water is also a common method of water treatment. It is interesting to note that around this time, although not mentioned by Molesworth, cathodic protection using zinc plates was recommended by some sources, although they recognized that regular replacement was necessary due to the high corrosion rate of zinc in hot water. From the late nineteenth century, the higher powered boilers in use had condensers and pure water could be used initially and reused, with only a small amount of makeup water required. An understanding of the chemistry and corrosion properties of water was also developing at this time and these principles were applied to boiler water. The importance of keeping the water as pure as possible and of minimizing oxygen content was passed on to boiler operators. However, the idea that additives could be beneficial was still some way off. Additives were introduced in the 1920s, when steam locomotives were dosed with enough caustic soda to make the water slightly alkaline to reduce the risk of acid corrosion but close enough to neutral to minimize scaling. By the 1930s, it was realized that adding other appropriate chemicals to the feed water meant these problems could be avoided in the first place. Appropriate corrosion and scale inhibitors were developed and widely used for treating boiler water. Other industrial waters, such as cooling water for heat exchangers, were treated on a more scientific basis at about the same time. As with boiler water, scale was the main enemy and similar treatments to boiler water - use of soft water, slight alkalinity, tannin and regular blow down - were the main treatments. Development of the first scaling index by Langelier in the 1930s further expanded scientific understanding of the fundamentals. Over the next decades, investigations into scaling and corrosion processes, along with the “new “problem of biological fouling, brought new products and practices in use today to control the often conflicting requirements to prevent corrosion, scaling and fouling. 24 Chapter 12 The Birth of Corrosion Science Not far from Piccadilly Circus in London’s West End, stands the impressive Royal Institution building, opened in 1799 for the advancement of science. In this building, two of the most eminent scientists of the nineteenth century made significant contributions to corrosion science and technology, including work on the fundamentals of galvanic corrosion, discovery of cathodic protection and creation of the terminology for corrosion reactions still in use today. Right from the start, the Royal Institution considered it important to reach out to the general public to show the importance of science in all works of life. It employed the young chemist Humphry Davy (1778-1829) in 1801 to stage entertaining and spectacular demonstrations of science in public lectures. These became one of the most popular events in London, and Albemarle Street became its first one way street. Davy also established a laboratory for scientific research and received a knighthood for his invention of the miner’s safety lamp. But less well known were his researches into electrochemistry. Following Galvani’s discovery of galvanic action in Bologna, Italy in 1780 and Volta’s invention of the electric battery in the late 1790s, Davy set about building the largest battery in the world at that time with 2000 pairs of plates. Using this, he managed to isolate elements such as sodium, potassium, calcium and other reactive elements for the first time. He showed there was a relationship between chemical reactivity and electricity, and produced what is probably the first Galvanic Series, which he described as “the different substances are arranged according to the order of their known galvanic powers, 25 [and] will shew [sic] how intimately chemical agencies are related to the production of galvanism.” Davy recognised the importance of dissimilar metals in corrosion behaviour: “Iron nails soon wear out when used to attach copper sheeting to ships and iron pins employed to attach lead to roofs of buildings rust with great rapidity, which is owing to chemical operations being increased by the electrical energy of contact.” He was asked to look at the corrosion of copper cladding used to prevent worms attacking the hulls of wooden naval ships. In 1824, Davy found that attaching iron or zinc ‘protectors’ prevented copper corrosion, pioneering cathodic protection. Unfortunately, this major discovery was a practical failure because copper needs to corrode to prevent formation of marine growth. But Davy can be considered as the father of cathodic protection, and an important pioneer in understanding of corrosion. Davy’s successor at the Royal Institution became an even more famous scientist. Michael Faraday (1791-1868) is rightly remembered for his discoveries in electro-magnetism leading to development of the electric motor and electric generator. But Faraday also carried out work in electro-chemistry, initially as assistant to Sir Humphrey Davy. His most important discovery in this area was his eponymous laws of electrolysis which showed the relationship between current and amount of metal corroded. It is ironic that the most useful mathematical formula in corrosion science was discovered by Faraday, as he had left school at age twelve and was always self-conscious about his lack of mathematical prowess. He made other contributions. After discussions with Cambridge polymath, William Whewell, he coined the nomenclature of electro-chemistry with which we are familiar today (electrode, anode, cathode, ion, anion, cation, electrolysis and electrolyte). In 1836, he carried out experiments on iron passivity, noting that iron placed in concentrated nitric acid did not corrode while rapid dissolution was observed in dilute nitric acid. 26 Faraday is also probably the first to suggest the combined use of protective coatings and cathodic protection for marine structures, a technique widely used today, when asked for advice on protecting iron piles for lighthouses: “Though iron be a body very subject to the action of sea water, it does seem … that it may be used to advantage in marine constructions intended to be permanent, especially if the joint effects of preserving coats and voltaic protectors were applied.” The Royal Institution is a worthy place to conclude this series as it brings us back to the most corrosion resistant metal – iridium – discussed in the first article. According to the Royal Society of Chemistry, Smithson Tennant, the discoverer of iridium in 1803, announced his finding … at the Royal Institution. 27 Tales of Metal Resilience – Robert A Francis An iron dagger found in Tutankhamun’s tomb. A water pipeline in the Australian outback. An ancient iron pillar outside India’s capital. An art deco architectural masterpiece in New York. What do these disparate items have in common? They are all examples showing it is possible to prevent corrosion which destroys tonnes of metal around the world. This publication provides twelve short vignettes covering these and other instances of metal resilience – victories in our long-standing fight against the ravages of corrosion. These examples are from around the world (and beyond) and from the distant past as well as present. It is designed to show that corrosion can be defeated and that corrosion science and engineering is as interesting, historic and artistic as any other. Rob Francis is recently retired corrosion and coating consultant with 50 years’ academic, industrial and consulting experience in corrosion and protective coatings. Dr Francis has a B.Sc. in metallurgy and a Ph.D. in corrosion science. He has authored or co-authored over forty technical papers or presentations on corrosion and coatings. He is also active on a number of standards committees in Australia and overseas.