Iron is possibly the most used and most important metal today. Ancient Egyptians learned how to use iron before the First Dynasty, which started pre 3000 BC. The probability is that the Egyptians found iron in meteorites, some of which are very rich in iron.
As they fashioned meteoric iron into small tools and ornaments they had no inkling iron would turn out to be the fourth most abundant element in the earth on which they stood.
From around 1200 BC, people learned how to smelt iron metal from earth ores and use it for making many diverse products. It is still and probably always will be called the Iron Age. Iron was common to early Asian civilisations. In Delhi, India, a pillar made out of iron built in 415 AD still stands. It weighs 6 500 kg and remains in remarkably good condition after nearly 1 600 years. Early Chinese civilisations also knew about iron. Workers there learned to work with iron as early as 200 BC. A number of iron objects, including cannons, remain from the Han period, from 202 BC to 221 AD. It was not until 1786, however, that scientists determined quantitatively what it was in steel that made it a more useful metal than iron. Three researchers, Gaspard Monge (1746-1818), C A Vandermonde, and Claude Louis Berthollet (1748-1822), found that small amounts of carbon mixed with iron, produced alloys which we know as steel.
The term ‘steel’ actually refers to a wide variety of products. Various forms of steel all contain iron and carbon. They also contain one or more other elements, such as silicon, titanium, vanadium, chromium, manganese, cobalt, nickel, zirconium, molybdenum, and tungsten. Two other steel-like products are cast iron and wrought iron. Cast iron is an alloy of iron, carbon, and silicon. Wrought iron contains iron and one or many other elements. In general, wrought iron contains very little carbon.
Corrosion of iron
Huge amounts of money are lost every year because of corrosion. Much of this loss is due to the corrosion of the iron content of steel, although many other metals may corrode as well. The problem with iron and other metals is that the iron oxides formed by oxidation do not firmly adhere to the surface of the metal. They flake off easily leaving behind ‘pitted’ surfaces. Extensive pitting eventually causes structural weakness and disintegration of the metal. Other metals such as aluminium and copper form very tough oxide coatings which bond strongly to their surfaces preventing them from further exposure to oxygen and corrosion. Elements such as silicon, chromium and nickel used in steel alloys provide varying degrees of resistance to corrosion by forming protective layers on iron surfaces. Other types of surface coatings include galvanising, provide long term protection against corrosion on steel surfaces because oxidised exterior layers of these materials [the zinc in galvanising] do not flake off but continue to form an integral part of the protective coating.
The word ‘corrosion’ is often wrongly used to describe the reactions of many chemicals, particularly acidic chemicals, on iron and other metal surfaces. This incorrect usage is also associated with degradation of metal surfaces by micro-organisms, particularly the well-known anaerobic Sulphate Reducing Bacteria (SRB) which does not attack metals directly but produces hydrogen sulphide which hydrolyses with any water present forming sulphurous and sulphuric acids. Corrosion as such, occurs in the presence of moisture. For example when iron is exposed to moist air, it reacts with oxygen to form rust.
The amount of water [X H2O] in the iron oxide (ferric oxide) varies as indicated by the letter ‘X’. The amount of water present also determines the colour of rust, which may vary from black to yellow to orange brown. The formation of rust is a very complex process which is thought to begin with the oxidation of iron to ferrous (iron ‘+2’) ions.
Both water and oxygen are required for the next sequence of reactions. The iron (+2) ions are further oxidised to form ferric ions (iron ‘+3’) ions.
The electrons provided from both oxidation steps are used to reduce oxygen.
The ferric ions then combine with oxygen to form ferric oxide [iron (III) oxide] which is then hydrated with varying amounts of water. The overall equation for rust formation may be written as:
The formation of rust can occur at some distance away from the actual pitting or erosion of iron as illustrated in figure 1. This is possible because the electrons produced via the initial oxidation of iron can be conducted through the metal and the iron ions can diffuse through the water layer to another point on the metal surface where oxygen is available. This process results in an electrochemical cell in which iron serves as the anode, oxygen gas as the cathode, and the aqueous solution of ions serve as a ‘dissolved - salts bridge’.
The involvement of water accounts for the fact that rusting takes place more rapidly in moist conditions as compared to a dry environment such as a desert. Many other factors affect the rate of corrosion. For example, the presence of dissolved salts in the water greatly accelerates the rusting of metals. This is due to the fact that the dissolved salts increase the conductivity of the aqueous solution formed at the surface of the metal which enhances the rate of electrochemical corrosion.
Protecting normal non-stainless carbon steel surfaces against exterior corrosion from ordinary rainwater and atmospheric oxygen has long been done with paints. Extra protection for more aggressive environments is provided by tougher coatings such as epoxies. Internal wetted areas of steel machinery and pipes are another matter entirely, as although some machines, such as galvanised cooling towers and evaporative condensers, have internal water passage anti-corrosion protection. Most steel water pipes do not. Therefore, whenever non-stainless steel pipes are installed for open evaporative cooling water circuits, closed cooling water circuits, and hot water circuits, the potential for iron corroding wetted surfaces must be addressed.
In evaporative water circuits, potential corrosion is normally much lower than in closed circuits due to higher pH levels resulting from increased dissolved calcium and magnesium carbonate salts due to these circuits running at cycles of concentration of six or higher. Regular bleed-off of circulating water also prevents build up of contamination by small amounts of iron corrosion products flaking off from steel surfaces from time to time. Chemical treatment of open water circuits is normally done by automatic dosing into supply make-up water so it is easy to maintain minimum levels of anti-corrosion chemicals.
Closed water circuits are seldom equipped with automatic chemical dosing systems and because, theoretically, there are no circulating water losses, an initial one-off dosage of chemicals is expected to prevent, or at least reduce to an extremely low level, the formation and progress of electrochemical corrosion cells as shown in figure 1 for periods ranging from 90 days to a year. From a chemical point of view, this is possible provided there are no water leaks, sufficient chemicals are dosed which do not react with the water, the internal wetted steel surfaces are genuinely clean to start off with, and there is a way of getting the required chemicals into the circulating water without any delay.
Current practice in South Africa for monitoring closed water circuits which, in HVAC installations are either hot or cold circuits, is to take samples of circulating water and test them at regular intervals which is normally every 90 days. Since the water circuits are closed, any water contamination showing up in the tests must be internally generated, coming from the wetted steel surfaces. Undissolved or suspended particles are virtually always iron corrosion products – the iron oxides and hydroxides referred to earlier. Low levels of iron found are assumed to indicate correspondingly low rates of iron corrosion. If corrosion coupons were originally installed they can be cleaned and weighed making calculations of amounts of steel lost per year possible. A further indication of corrosion is a significant decrease in the pH of the circulating water which is explained by the end of the previously detailed overall equation for rust formation [- - - - -+ 8H+[aq]] adding hydrogen ions to the water. Lower pH water holds more iron in solution which further complicates linking test results for iron in water with actual corrosion rates of steel. On a practical level therefore, when iron test results show high iron concentrations – upwards from about five milligrams per litre – calculations of the rate of metal loss from weight loss data of corrosion coupons determines what corrective actions are needed. The generally accepted maximum reduction in the thickness of a representative steel coupon is 0,02 mm per year.
Iron corrosion in steel is irreversible so it is imperative to reduce it as much as is practically possible in the continuously increasing number of cooling and heating water circuits in which ordinary non-stainless steels are used. Fifty years ago, when low dosages of chromate anti-corrosion chemicals were permitted, corrosion rates of below 0,01 mm per year were the norm. The next best anti-corrosion agent, zinc, joined chromium on the banned list in 1980. Today, the two most widely used anti-corrosion treatment systems are based on two other metals, molybdenum and boron, both of which are under scrutiny for any evidence of accumulative toxicity. Of the two, boron is the more popular in the well known ‘boron nitrite’ form which does reduce corrosion rates to below 0,02 mm per year when properly applied and maintained. However, boron nitrite has become a relatively high cost item in maintenance budgets for HVAC installations which feels out of place by both users and suppliers for a straightforward chemical treatment system which has been unaltered for over 60 years. It seems, in fact, puzzling that at present there is no indication whatsoever of any potential for improved replacements for the molybdate/boron products for such a large and growing market compared to, for example, what has been done with refrigerants in this era of continuously developing technology.