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Richard Seligman The cover photo depicts galvanically induced corrosion of stainless steel caused by the deposition of active carbon used as a decolorizing agent for the sugar solution in contact with the metal. The handbook provides a detailed look at the various types of corrosion, the effects of the most common corrosive agents with respect to a variety of materials and an overview of how to cost effectively select the materials of construction most suited for your particular application.
It is worth devoting a few paragraphs to explain the basic Ferritic or intermediate structure metallurgy of stainless steels. Unfortunately, they lacked the ductility to undergo extensive fabrication, and they could not be welded.
Numerous workers tried to overcome these deficiencies by the addition of other alloying elements, to produce a material where the ferrite was transformed to austenite another metallurgical phrase — that was stable at room temperature.
Soft stainless steels that were ductile both before and after welding were developed in Sheffield, England then the heart of the British steel industry — exploiting scientific work undertaken in Germany.
Probably the next major advance in the development of stainless steels was the discovery that relatively small additions of molybdenum had a pronounced effect on their corrosion resistance, greatly enhancing their ability to withstand the effects of mineral acids and other corrodents such as chloride solutions.
Needless to say, from these early developments, there has been tremendous growth in the production facilities and the number of grades of stainless steel available. Unless indicated otherwise, all values are maximum. Although stainless steel offers excellent resistance to a wide range of chemical and non-chemical environments, it does not offer immunity to all.
In the case of the food industry, these are notably anything containing salt, especially low pH products. There was, therefore, a demand by industry to develop more corrosion resistant materials, and these are finding an increased use in the food industry for certain specific processing operations.
Still, further increases in the three aforementioned alloying elements result in the nickel alloys. The classification of an alloy is generally under the heading of the major constituent. There are a large number of these alloys, but those of primary interest to the food industry are shown in Table 2, together with their compositions. In general terms, it will be noted that the increase in nickel content is accompanied by an increase in chromium and molybdenum.
As previously stated, this element is particularly effective in promoting corrosion resistance. Just like insurance, you only get what you pay for — and generally speaking, the higher the corrosion resistance, the more expensive the material. In fact, the differential between type stainless steel and a high nickel alloy may be as much as 20 times, depending on the market prices for the various alloying elements — which fluctuate widely with the supply and demand position.
This resistance is attributable to the easy and rapid formation of a thin, continuous, adherent oxide film on exposed surfaces. As aluminum salts formed by corrosion are colorless, tasteless, and claimed to be non-toxic — the metal is easy to clean, inexpensive, light, and has a high thermal conductivity.
It still is used quite extensively in certain areas of food manufacture and distribution. However, the case is far from proven, and it is not clear if the increased levels of aluminosilicates are due to a high intake of aluminum per se or other factors such as dietary deficiency of calcium. For many years, aluminum was extensively used for containment vessels in the dairy and brewing industry.
Many of these original vessels are still in use in some of the smaller, privately owned breweries in the United Kingdom. Although large fermenting vessels and storage tanks now tend to be fabricated from stainless steels, there is still widespread use of aluminum for beer kegs, beer cans, and a miscellany of small-scale equipment where the resistance of aluminum is such that it imparts no charge or modification of flavor, even after prolonged storage.
While still used for holding vessels and some equipment when processing cider, wines, and perry, prolonged contact is inadvisable due to the acidity of the sulphites employed as preservatives for these products — inadvisable, that is, unless the surface of the metal has been modified by anodizing or has been protected with a lacquer. In the manufacture of preservatives, aluminum is still employed for boiling pans, with the presence of sugar appearing to inhibit any corrosion.
In the field of apiculture, it has even been used for making prefabricated honeycombs which the bees readily accepted. Extensive use is made of aluminum and the alloys in the baking industry for baking tins, kneading troughs, handling equipment, etc.
In other areas of food manufacture and preparation, the use of aluminum virtually extends over the whole field of activity — butter, margarine, table oils and edible fats, meat and meat products, fish and shellfish, certain sorts of vinegar, mustards, spices; the list is almost endless.
No mention has so far been made of the application of this metal in the dairy industry. It still has limited application mostly in the field of packaging, e.
The uses of aluminum in the food industry so far mentioned have tended to be for equipment used in batch operation, hand utensils, and packaging. There are probably three major factors which have mitigated against its more widespread use, not only in the dairy industry but in brewing and many other branches of food processing. Modern, highly automated plants operating on a continuous or semi-continuous basis employ a wide variety of materials of construction.
The commercial availability of stainless steels, their ease of fabrication, strength, ease of maintenance, appearance, and proven track record of reliability. Since modern plants operate on a semi-continuous basis with much higher levels of fouling, cleaning regimes require strongly alkaline detergents to which aluminum has virtually zero corrosion resistance. However, the demise of copper as a material of construction is largely attributable to the toxic nature of the metal and its catalytic activity in the development of oxidative rancidity in fats and oils.
Even at the sub-part per million level, copper in vegetable oils and animal fats rapidly causes the development of off-flavors. In equipment where high levels of liquid turbulence are encountered, e. Nevertheless, there is an area of the beverage industry where copper is still the only accepted material of construction, i.
It is also used in the distillation of spirits such as rum and brandy. A lot of old copper brewing equipment, such as fermenting vessels and wort boilers, is still in use throughout the world. More and more, food processors are accepting the use of titanium as an alternative, in the full knowledge that it offers corrosion immunity to the more aggressive foodstuffs and provides a long term solution to what was an on-going problem with stainless steels.
Although relatively expensive six to seven times the cost of stainless steel , being a low density material offsets this price differential for the raw material by almost half.
It is ductile and fabricable using normal techniques, although welding it does require a high degree of expertise. Cadmium, used as a protective coating for carbon steel nuts and bolts, was favored at one time. However, the high toxicity of cadmium compounds has come under increased scrutiny from many health regulatory bodies, and now cadmium plated bolting is not permitted in food factories.
Denmark and Sweden have totally banned the import of cadmium plated components into their countries and many other countries are likely to follow suit. A piece of equipment which prematurely fails by corrosion is as badly designed as one in which the materials have been over-specified. Unfortunately, all too often the functional requirements for a piece of equipment are analyzed in a somewhat arbitrary manner. And the basic cost of the material often tends to outweigh other equally important considerations.
Figure 3 shows the primary criteria which must be considered in the initial selection process. When considering corrosion resistance, the operational environment is the obvious one. But, the other point must be whether the material will also offer corrosion resistance to the chemicals used for cleaning and sanitizing. For example, there is no point in considering a high nickel alloy when a standard series stainless steel, at a lower cost, will be perfectly satisfactory. Many steel producers will require a minimum order of, say, three tons for a non-standard material.
Clearly, the equipment manufacturer is not going to buy this large quantity when the job that is to be done may only require the use of one ton of material. Equipment located outside must be resistant to environmental weathering, and therefore, may require the application of protective sheathing — which could double the basic material cost.
How long will the equipment operate without the need for major servicing? When all these interrelated criteria have been considered, the long list of possible starters will have been reduced to maybe one or two. Also, through the selection process some of those materials initially rejected due to their high cost, for example, may have to be reconsidered because of other factors.
There are several definitions of corrosion. For example, Fontana Ref. However, the primary object of this paper is to deal with corrosion of metals, in particular stainless steels, and how this corrosion can be classified. There are two basic forms of corrosion — wet and dry.
Dry corrosion is concerned with the oxidation of metals at high temperatures and is outside the scope of this text. Wet corrosion occurs in aqueous solutions, or in the presence of electrolytes, and is an electrochemical process.
In fact, steel is not corroded by chlorine since steel is the material used for storing liquid chlorine. However, in the presence of even trace quantities of moisture, chlorine rapidly attacks steel — and for that matter, most metals.
The corrosion of metals involves a whole range of factors. These may be chemical, electrochemical, biological, metallurgical, or mechanical — acting singly or conjointly. Nevertheless, the main parameter governing corrosion of metals is related to electrochemistry.
Electrochemical principles, therefore, are the basis for a theoretical understanding of the subject. It is not proposed to discuss, in depth, the electrochemical nature of corrosion — but should further information be required, several excellent texts are available Ref. If the two metals are electrically connected, then, because of this potential difference, a flow of current occurs.
As the corrosion process is an electrochemical phenomenon and dissolution of a metal involves electron flow, the corrosion rates for the two metals are affected.
Generally, the corrosion rate for the least corrosion resistant is enhanced while that of the more corrosion resistant is diminished. In simple electrochemical terms, the least resistant metal has become anodic and the more resistant cathodic. This, then, is galvanic or dissimilar metal corrosion. The magnitude of the changes in corrosion rates depends on the so-called electrode potentials of the two metals; the greater the difference, the greater the enhancement or diminution of the corrosion rates.
It is possible to draw up a table of some commercial alloys which ranks them in order of their electrochemical potential. Such a table is known as the galvanic series. This galvanic series relates to tests in unpolluted sea water, although different environments could produce different results and rankings.
When coupled, individual metals and alloys from the same group are unlikely to show galvanic effects which will cause any change in their corrosion rates. Probably the most common form of unintentional galvanic corrosion is on service lines where brass fittings are used on steel pipelines — the steel suffering an increase in corrosion rate at the bimetallic junction. One of the worst bimetallic combinations is aluminum and copper. An example of this is in relation to aluminum milk churns used to transport whey from Gruyere a cheese manufacturer in Switzerland , where copper is used for the cheese-making vats and the whey picks up traces of this metal.
The effect on the aluminum churns, which are internally protected with lacquer that gets worn away through mechanical damage, is pretty catastrophic. Note the large pit associated with the weld splatter. Another somewhat unique example of galvanic corrosion is related to a weld repair on a stainless steel storage vessel. Welding consumables containing molybdenum had been employed to effect the repair.
Although it is unusual for the steels to be sufficient to initiate galvanic corrosion, the environmental factors in this particular case were obviously such that corrosion was initiated Figure 5. As stated, this is somewhat unique and it is not uncommon for welding consumables to be used for welding stainless steel with no adverse effects.
As a practice, however, it is to be deprecated and the correct welding consumables should always be employed. Not all galvanic corrosion is bad; galvanic corrosion is used extensively to protect metal and structures by the use of a sacrificial metal coating.
When the galvanizing film is damaged, the zinc galvanically protects the exposed steel and inhibits rusting. Similarly, sacrificial anodes are fitted to domestic hot water storage tanks to protect the tank. It is the most common form of corrosion encountered with the majority of metals, a classic example being the rusting of carbon steel.
Insofar as the corrosion occurs uniformly, corrosion rates are predictable and the necessary corrosion allowances are built into any equipment. In the case of stainless steels, this form of corrosion is rarely encountered.
Corrodents likely to produce general attack of stainless steel are certain mineral acids, some organic acids and high strength caustic soda at concentrations and temperatures well in excess of those ever likely to be found in the food industry.
The same remark applies to cleaning acids such as nitric, phosphoric and citric acids — but not for sulfuric or hydrochloric acids — both of which can cause rapid, general corrosion of stainless steels. Hence, they are not recommended for use, especially where corrosion would result in a deterioration of the surface finish of process equipment. The behavior of both and stainless steels when subjected to some of the more common acids that are encountered in the food industry is graphically illustrated by Figure 6.
These iso-corrosion graphs, i. They are of no value when the corrosion mode is one of the other forms which will be defined, such as pitting or crevice corrosion. It is characteristically encountered with metals and alloys which rely on a surface oxide film for corrosion protection, e. The crevices can be inherent in the design of the equipment e.
Crevice contact points of a heat exchanger plate. Any non-metallic material which is porous and used as a gasket, for example, is particularly good or bad! Fibrous materials which have a strong wicking action are notorious in their ability to initiate crevice attack. Similarly, materials which have poor stress relaxation characteristics, i. Although used for gasketing, P.
On the other hand, elastomeric materials are particularly good insofar as they exhibit elastic recovery and have the ability to form a crevice-free seal. However, at elevated temperatures, may rubbers harden.
In this condition, they suffer the deficiencies of non-elastomeric gasketing materials. Artificial crevices can also be created by the deposition of scale from one of the process streams to which the metal is exposed. Much research has been done on the geometry of crevices and the influence of this on the propensity for the initiation of crevice corrosion Ref. However, in practical terms, crevice corrosion usually occurs in openings a few tenths of a millimeter or less, and rarely is encountered where the crevice is greater than 2 mm 0.
Until the s, crevice corrosion was thought to be due to differences in metal ion or oxygen concentration within the crevice and its surroundings. While these are factors in the initiation and propagation of crevice corrosion, they are not the primary cause. Current theory supports the view that through a series of electrochemical reactions and the geometrically restricted access into the crevice migration of cations — chloride ions in particular — occurs.
This alters the environment within, with a large reduction on pH and an increase in the cations by a factor as much as ten. The pH value can fall from a value of, say, seven in the surrounding solution to as low as two within the crevice. As corrosion is initiated, it proceeds in an autocatalytic manner with all the damage and metal dissolution occurring within the crevice.
Corrosion results in significant loss of metal under the surface of site of initiation. As a result, deep and severe cutting of the metal occurs see Figure 8. The time scale for initiation of crevice corrosion can vary from a few hours to several months, and once initiated, can very rapidly progress.
The difficulty of attaining this will be appreciated by reference to Figure 8, where the entrance to the crevice is only 0. While methods for combating the onset of crevice corrosion can be deduced from the foregoing text, a reiteration of some of the Fig. Note the deep undercutting which is typical follows: of chloride-induced attack on stainless steel.
However, certain pieces of equipment are by virtue of their design highly creviced. In such cases, it is necessary to recognize the potential corrosion risk and select the materials of construction which will resist the initiation of crevice corrosion by the environment. Similarly, cleaning and sanitizing regimes must be developed to avoid the onset of attack. In the case of stainless steels, although there are several ionic species which will initiate the attack, by far the most common are solutions containing chloride.
The presence of salt in virtually all foodstuffs highlights the problem. Low pH values also enhance the propensity for initiation of attack. Other environmental factors, such as temperature and the oxygen or dissolved air content of the process stream, all play a role in the corrosion process. Because the presence of oxygen is a prerequisite for the onset of crevice corrosion and many other forms of attack , in theoretical terms complete removal of oxygen from a process stream will inhibit corrosion.
In practice, however, this is difficult to achieve. Only in equipment where complete and effective deaeration occurs, such as a multiple effect evaporator operating under reduced pressure, will the beneficial effect of oxygen removal be achieved. Stainless steels containing molybdenum , have a much higher resistance to crevice corrosion than alloys without this element , , The higher the molybdenum content, the greater the corrosion resistance.
For particularly aggressive process streams, titanium is often the only economically viable material to offer adequate corrosion resistance. It is a form of extremely localized but intense attack, insidious insofar as the actual loss of metal is negligible in relation to the total mass of metal which may be affected. Nevertheless, Fig. The pits can be small and sporadically distributed over the metal surface Figure 9 or extremely close together, close enough in fact to give the appearance of the metal having suffered from a general attack.
In the case of stainless steels, environments which will initiate crevice corrosion will also induce pitting. As far as the food industry is concerned, it is almost Fig. Many theories have been developed to explain the cause of the initiation of pitting corrosion Ref. This results in ionic migration and the development of an electrochemical cell. There is, however, no unified theory which explains the reason for the film breakdown. Evans Ref. However, propagation of the pit proceeds by a mechanism similar to that occurring with crevice corrosion.
Like crevice corrosion, the pits are often undercut and on vertical surfaces may assume an elongated morphology due to gravitational effects Figure The onset of pitting corrosion can occur in a matter of days but frequently requires several months for the development of recognizable pits. This makes the assessment of the pitting propensity of a particular environment very difficult to determine, and there are no short-cut laboratory testing techniques available.
Another chemical method involving ferric chloride determines the temperature at which the solution will cause pitting within a hour period, the results being expressed as the critical pitting temperature or CPT Ref. Electrochemical methods have also been used.
As with crevice corrosion, alloy composition has a profound effect on the resistance of a material to a pitting attack. Greene and Fontana Ref. However, unlike crystalline solids such as sugar or salt, metallic crystals can be deformed or bent without fracturing; in other words, they are ductile.
In the molten state, the atoms in a metal are randomly distributed but on cooling and solidification, they become arranged in crystalline form. Because crystallization occurs at many points in the solidification process, these crystals or grains are randomly orientated and the region where they meet are grain boundaries. In thermodynamic terms, the grain boundaries are more susceptible to corrosion attack because of their higher free energy.
Although, in practice the free energy of the grain boundary and the main crystals or grains in a homogeneous alloy are too small to make a significant difference. However, when the metal or alloy has a heterogeneous structure, preferential attack at or adjacent to the grain boundaries can occur. This is intergranular corrosion as shown in Figure It is generally agreed that this is due to chromium combining with carbon to form chromium carbide, which Fig.
As such, this zone depleted in chromium bears little similarity to the main metal matrix and has lost one of the major alloying elements on which it relied for its original corrosion resistance.
The lowering of corrosion resistance in this zone is so great that sensitized materials are subject to attack by even mildly corrosive environments. As supplied from the steel mills, stainless steels are in the so-called solution annealed condition, i. Therefore, this zone may be susceptible to the development of intergranular carbide precipitates. Because the formation of chromium carbides is a function of time, the longer the dwell time in the critical temperature zone, the greater the propensity for carbide formation.
Hence, the problem is greatest with thicker metal sections due to the thermal mass and slow cooling rate. By rapidly cooling or quenching the steel from this temperature, the original homogeneous structure is re-established and the original corrosion resistance is restored. The first stainless steels were produced with carbon contents of up to 0.
In consequence, the carbon levels were reduced to 0. Although this move alleviated the problem, it was not wholly successful, particularly when welding thicker sections of the metal. Solution annealing of the fabricated items was rarely a practical proposition and there was a need for a long term solution. It was shown that titanium or niobium comumbium had a much greater affinity for the carbon than chromium, and by additions of either of these elements, the problem was largely overcome.
The titanium or niobium carbides which are formed remain dispersed throughout the metal structure rather than accumulating at the grain boundaries. Grade is a type 18Cr, 8Ni with titanium added as a stabilizing element, while grade contains niobium. By far, the most commonly used is , grade being specified for certain chemical applications.
Modern steel-making techniques such as AOD air-oxygen decarburization were developed to reach even lower levels of carbon, typically less than 0. These are commercially available and routinely specified where no sensitization can be permitted. With these advances in steel-making technology, even the standard grades of stainless steels have typical carbon levels of 0.
The morphology of this type of failure is invariably a fine filementous crack which propagates through the metal in transgranular mode. Frequently, the crack is highly branched as shown in Figure 12, although sometimes it can assume a single crack form. Factors such as metal structure, environment, and stress level have an effect on crack morphology. The disturbing feature of SCC is that there is virtually no loss of metal, and frequently it is not visible by casual inspection and is only apparent Fig.
Another characteristic of SCC in stainless steels is that once detected, repair by welding is extremely difficult. Crack propagation frequently occurs below the surface of the metal and any attempt to weld repair results in the crack opening up and running ahead of the welding torch. The only practical method of achieving a satisfactory repair is to completely remove the affected area with a cm six to nine inches allowance all around the area of visible damage and replace the section.
Even then, there is no guarantee that the damaged zone has been entirely removed. In most cases, there are three prerequisites for the initiation of SCC. Furthermore, it has been observed that a corrosion pit can act as a stress raiser and a nucleation site for SCC.
High strength caustic soda at elevated temperatures will also induce SCC, but the concentrations and temperatures required are well in excess of those ever likely to be encountered. Furthermore, the crack morphology is intergranular rather than transgranular. Therefore, it is worth considering the practical approach to its elimination from equipment.
On small pieces of equipment, residual stress from manufacturing operations can be removed by stress relief annealing. For large pieces of equipment such as 80 storage vessels, this approach is clearly impractical. However, this is a somewhat 20 impractical and uneconomic approach. This, too, is not a test solution. As for the matter of corrosive species, it is questionable if anything can be done about elimination.
CK As shown by Copson Ref. Increasing the nickel content Fig. Data points have been omitted for clarity. Compare the size of grains in the weld with those in the parent material on the left. But of course, this approach incurs not only the increased cost of the nickel but also the added penalty of having to increase the chromium to maintain a balanced metallurgical structure.
The more effective approach is by reducing the nickel content of the alloy. Molybdenum may or may not be present, depending on the environment for which the alloy has been designed. It is only with the advent of modern steel-making technology, particularly in relation to the lower carbon levels that can be achieved, that these alloys have become a commercially viable proposition.
Because of their low carbon content, typically 0. The alloys are almost twice as strong as the austenitic stainless steels, and are ductile and weldable. From a general corrosion standpoint, they are comparable with, or marginally superior to, their series equivalent; but from an SCC standpoint in test work and from field experience, they offer a resistance order of a magnitude better.
This alloy contains carbon at the 0. Furthermore, stabilizing elements such as titanium and niobium are also alloying additions which minimize the tendency for intergranular chromium carbide formation. The big disadvantage of these materials is their susceptibility to grain growth during welding Figure 15 which makes them extremely sensitive to fracture, even at room temperature.
Because pure fatigue is an in vacuo phenomenon, a more correct term is corrosion fatigue or environmental cracking, which is the modern expression and takes into account cracking where the corrosive factor has played a major role on the crack morphology.
Compare this with Figure Note the step-wise progression of the crack. The primary cause of corrosion fatigue is the application of fluctuating pressure loads to components which, while of adequate design to withstand normal operating pressures, eventually fail under the influence of cyclic loading.
The components can be of extremely rigid construction, such as homogenizer block or of relatively light construction, such as pipework. There are many potential sources of the fluctuating pressure, the most common of which are positive displacement pumps e.
Generally speaking, fatigue cracks are straight, without branching and without ductile metal distortion of the material adjacent to the crack. The one unique characteristic of fatigue cracks is that the crack face frequently has a series of conchoidal markings which represent the step-wise advance of the crack front — see Figure Although, as stated, corrosion fatigue cracks are generally straight and unbranched — where fatiguing conditions are in a potentially corrosive environment — the influence of the corrosive component may be superimposed on the cracks.
This can lead to branching of the cracks, and in the extreme case, the crack may assume a highly branched morphology which is almost indistinguishable from stress corrosion cracking. It is only when the crack face is examined under high-power magnification that it is possible to categorize the failure mode.
An example of this is shown in Figure 17 which has all the features of stress corrosion cracking, but the scanning electron micrograph Figure 18 clearly shows the step-wise progression of the crack front. The site for initiation of corrosion fatigue is frequently a discontinuity in the metal section of the component. This may be, for example, from a sharp change in diameter of a shaft where inadequate radiusing of the diametric change results in a high cyclic stress level through shaft rotation, and thus, an initiation point for fatigue.
The whole subject of fatigue and corrosion fatigue is complex. However, as far as food processing equipment is concerned, avoidance of fatigue failure is best achieved by avoiding pulsing and pressure peaks. This requires the use of well engineered valving systems and avoiding the use of positive displacement pumps. Where this is impractical, provisions should be made to incorporate pulsation dampers which will smooth out the pressure peaks and minimize the risk of fatigue failures.
Although the cant and wrought materials have similar, but not identical compositions with regard to their chromium and nickel contents, metallurgically they have different structures.
The ferrite is necessary to permit welding to the castings, to avoid shrinkage cracking during cooling from the casting temperature. It also acts as nucleation sites for the precipitation of chromium carbides which will invariably be present, as it will not always be possible to solution anneal the cast components. Because the crystallographic structures of the ferrite and austenite differ austenite being a face center cubic and the ferrite being a body center cubic , the ferrite has — thermodynamically speaking — a higher free energy which renders it more susceptible to attack, particularly in low pH chloride containing environments such as tomato ketchup and glucose syrups.
Although the problem is not so severe when the ferrite occurs as isolated pools, when present as a continuous network, propagation of the corrosion occurs along the ferrite with the austenite phase being relatively unaffected — Figure Because the products of corrosion are not leached out from the corrosion site and are more voluminous than the metal, corroded castings frequently assume a blistered or pockmarked Fig.
The common environments CF8M showing preferential attack of the ferrite phase. Therefore, this form of damage is frequently also present — Figure Depending on the method used to make the casting, the surface of the castings can be chemically modified, which reduces the corrosion resistance. Small components are usually cast by either the shell molding process or produced as investment castings. In the shell molding process, the sand forming Fig.
Note the pock-like corrosion sites. This results in the metal adjacent to the mold having an enhanced carbon level with the formation of intergranular carbide precipitates. Hence, there is a susceptibility to intergranular attack and other forms such as crevice, pitting, and stress corrosion cracking.
Methods of overcoming this include solution annealing or machining off the carburized skin of metal. With investment castings the mold is made of zircon sand zirconium silicate , and fired at a high temperature to remove all traces of organic material and wax, which is used as a core in the mold making process.
They do not, therefore, have this carburized layer and offer a much lower resistance to surface corrosion. One method of overcoming this is to incorporate a small amount, typically 0.
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