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chemical process. It should be noted that the "aqueous" component of the system may be present in only trace quantities (eg, present as moisture); the classical example is the corrosion of steel by chlorine gas. 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. In fact, electrochemical techniques are now the standard method for investigating corrosion although the standard "weight loss" approach still provides invaluable data. It is not proposed to discuss in depth the electrochemical nature of corrosion but should further information be required, several excellent texts are available (7,8).
Forms of Corrosion
Wet corrosion can be classified under any of eight headings, namely:
• Galvanic or bimetallic corrosion
• Uniform or general attack
• Crevice corrosion
• Pitting corrosion
• Intergranular corrosion
• Stress corrosion cracking
• Corrosion fatigue
• Selective corrosion (castings and free-matching stainless steels)
Galvanic Corrosion. When two dissimilar metals (or alloys) are immersed in a corrosive or conductive solution, an electrical potential or potential difference usually exists between them. 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 is 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 cath-odic. 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 that ranks them in order of their electrochemical potential. Such a table is known as the galvanic series. A typical one as shown in Table 3 is based on work undertaken by the International Nickel Company (now INCO Ltd.) at their Harbor Island, NC, test facility. This galvanic series relates to tests in unpolluted seawater, 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 that will cause any change in their corrosion rates.
The problem of dissimilar metal corrosion, being relatively well understood and appreciated by engineers, is
Table 3. The Galvanic Series of Some Commercial Metals and Alloys in Clean Seawater
Chlorimet 3 (62 Ni, 18 Cr, 18 Mo) Hastelloy C (62 Ni, 17 Cr, 15 Mo) 18/8 Mo stainless steel (passive) 18/8 stainless steel (passive) Chromium stainless steel 11-30% Cr (passive) Inconel (passive) (80 Ni, 13 Cr, 7 Fe) Nickel (passive) Silver solder Monel (70 Ni, 30 Cu) Cupronickels (60-90 Cu, 40-10 Ni) Bronzes (Cu-Sn) Copper
Chlorimet 2 (66 Ni, 32 Mo, 1 Fe) Hastelloy B (60 Ni, 30 Mo, 6 Fe, 1 Mn) Inconel (active) Nickel (active) Tin Lead
Lead-tin solders 18/8 Mo stainless steel (active) 18/stainless steel (active) Ni-resist (high-Ni cast iron) Chromium stainless steel, 13% Cr (active) Cast iron Steel or iron
Commercially pure aluminum (1100) Zinc
Magnesium and magnesium alloys
Active or anodic usually avoided in plant construction and in the author's experience, few cases have been encountered. 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 cheese manufacture (in Switzerland), where copper is used for the cheesemaking 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.
Another, somewhat unique, example of galvanic corrosion is related to a weld repair on a 304 stainless-steel storage vessel. Welding consumables containing molybdenum had been employed to effect the repair, and although it is most unusual for the potential difference between molybdenum and nonmolybdenum containing stainless steels to be sufficient to initiate galvanic corrosion, the environmental factors in this particular case were obviously such that corrosion was initiated (Fig. 5). As stated, this is somewhat unique and it is not uncommon for 316 welding consumables to be used for welding 304 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; indeed, galvanic corrosion is used extensively to protect metal and structures by the use of a sacrificial metal coating. A classic example is the galvanizing of sheet steel and fittings, the zinc coating being applied not so much because it does not corrode, but because it does. 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.
Uniform or General Attack. As the name implies, this form of corrosion occurs more or less uniformly over the whole surface of the metal exposed to the corrosive envi-
ronment. 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 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 304 and 316 stainless steels when subjected to some of the more common acids that are encountered in the food industry is graphically illustrated in Figure 6. These isocorrosion graphs, ie, lines that define the conditions of temperature and acid concentration that will produce a constant corrosion rate expressed in mils (0.001 in.) or mm loss of metal thickness per year, are used extensively by corrosion engineers in the material selection process when the form of corrosion is general attack. They are of no value whatsoever when the corrosion mode is one of the other forms that will be defined, such as pitting or crevice corrosion.
Crevice Corrosion. This form of corrosion is an intense local attack within crevices or shielded areas on metal surfaces exposed to corrosive solutions. It is characteristically encountered with metals and alloys that rely on a surface oxide film for corrosion protection, eg, stainless steels, titanium, aluminum.
The crevices can be inherent in the design of the equipment (eg, plate heat exchangers) or inadvertently created by bad design. Although crevice corrosion can be initiated at metal-to-metal surfaces (see Fig. 7), it is frequently encountered at metal to nonmetallic sealing faces. Any non-metallic material that is porous and used, for example,as a gasket, is particularly good (or bad!) for initiating this form of attack. Fibrous materials that have a strong wick-ing action are notorious in their ability to initiate crevice attack. Similarly, materials that have poor stress relaxation characteristics, ie, have little or no ability to recover their original shape after being deformed, are also crevice creators, as are materials that tend to creep under the influence of applied loads and/or at elevated temperatures. Although used for gasketing, PTFE suffers both these deficiencies. 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, many rubbers harden and in this condition, suffer the deficiencies of many 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. It is necessary, therefore, to maintain food processing equipment in a scale-free condition, especially on surfaces exposed to service fluids such as services side hot/ cold water, cooling brines, which tend to be overlooked during plant cleaning operations.
Much research work has been done on the geometry of crevices and the influence of this on the propensity for the initiation of crevice corrosion (9). 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 (0.08 in.).
Until the 1950s, 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 in pH and an increase in the cations by a factor of as much as 10. The pH value can fall from a value of, say, 7 in the surrounding solution to as low as pH 2 within the crevice. As corrosion is initiated, it proceeds in an autocatalytic manner with all the damage and metal dissolution occurring within the crevice and little or no metal loss outside. The confined and autocatalytic nature of crevice corrosion results in significant loss of metal under the surface of site of initiation. As a result, deep and severe undercutting of the metal occurs (see Fig. 8). The time scale for initiation of crevice corrosion can vary from a few hours to several months and, once initiated, can progress very rapidly. Stopping the corrosion process can be extremely difficult as it is necessary to remove all the trapped reactants and completely modify the occluded environment. The difficulty of attaining this will be appreciated by reference to Figure 8, where the entrance to the corroded region is only 0.5 mm (0.020 in.).
While methods for combating the onset of crevice corrosion can be deduced from the foregoing text, a reiteration of some of the more important precautions is not out of place, viz:
• Good-quality, crevice-free welded joints are always preferable to bolted joints
• Good equipment design (well-designed gasket sealing faces) that avoids unintentional crevices and does not permit the development of stagnant regions
• Frequent inspection of equipment and removal of surface deposits
• Use of good-quality rubber gaskets rather than absorbent packings
• Good gasket maintenance; replacement when hardened or damaged
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 that will resist the initiation of crevice corrosion by the environment. Similarly, cleaning and sanitizing regimes must be developed to avoid the onset of attack.
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