Other Forms of Corrosion

AU the other forms of corrosion of aluminium and aluminium alloys such as waterline corrosion, crevice corrosion, and stress corrosion can develop in water. They can be prevented by an appropriate selection of alloys, tempers, and possibly protections, as well as by an appropriate design that prevents hidden recesses and dead zones. 

There are many other types of corrosion that may occur (some quite esoteric). Two forms of corrosion not yet discussed include chelant corrosion and nonferrous metal corrosion. 

Organic chelant compounds, such as the sodium salts of ethylenedi-aminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) are commonly used in BW deposit control treatments, often in combination with phosphates. 

NOTE EDTA, NTA, and other chelants are also employed in various online and offline boiler plant cleaning formulations. 

Chelants have the ability to take waterborne metal ions such as Ca and Mg (but also Fe and Cu) and produce soluble coordinate bond complexes. Therefore, they can be employed for the control of FW hardness salts and other related problems. 

Chelants are particularly useful in maintaining very clean, deposit-free waterside conditions and can be employed in both FT and WT boilers. In practice, however, they are relatively indiscriminant in their reactions and under unsuitable conditions may seriously corrode carbon steel, copper, and copper alloy boiler components. Chelants also may react with any available oxygen under BW conditions and temperatures. 

The two basic types of corrosion discussed above form the general background to the subject. How, and to what extent, any particular object or structure corrodes also depends on other factors, in particular, on whether corrosion is uniform or not and on the effects of mechanical strain. These factors are interactive and in combination, their individual effects can be enhanced. 

Uniform corrosion, which involves progressive and uniform thinning of the metal, is the simplest and commonest form of corrosion. With appropriate engineering design, it can be controlled relatively easily. 

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The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

Dealloying can be reduced (as can any other form of corrosion) by good system operation and the judicious use of appropriate materials and chemical treatment. Specific categories needing attention follow. 

Graphitic corrosion has two distinct features that are useful in distinguishing it from other forms of corrosion. First, it affects an unusually limited number of metals the only metals commonly affected are gray cast iron and nodular cast iron. Second, metal that has experienced graphitic corrosion may retain its original appearance and dimensions. Consequently, graphitic corrosion frequently escapes detection. 

During design the life expeetaney, due to ereep or other forms of corrosion, should be estimated and examination or replacement planned. Cheap fittings, such as studs, bolts, and nuts, should be replaced in good time. Not to do so is penny-pinching and expensive in the end. 

The influence of moisture is fundamental, as it is with other forms of corrosion. Long-term contact tests with ponderosa pine, some treated with zinc chloride, in atmospheres at 30, 65 and 95% r.h. showed that at 30 and 65% r.h. plain wire nails were not very severely corroded even in zinc chloride-impregnated wood. At 95% r.h. plain wire nails were severely corroded, though galvanised nails were attacked only by impregnated wood. Brass and aluminium were also attacked to some extent at 95% r.h. Some concurrent outdoor tests at Madison, Wisconsin, showed that the outdoor climate there was somewhat more severe than a 65% r.h. laboratory test. 

Evidence of waterside deposits, plugging, pitting, and other forms of corrosion, plus leaks, cracks, bulges, blisters, overheating, surging,... 

This is a general name for a wide variety of corrosion processes that may be actively or passively influenced, or induced, by an even wider variety of microbiological organisms. The electrochemical reactions that occur always result in metal wastage, as with all other forms of corrosion the most active biocorrosion processes primarily involve sessile bacteria rather than planktonic bacteria, algae, or fungi. 

Repassivation processes have become an important subject in stress corrosion studies and also in other forms of corrosion, e.g., pitting corrosion and corrosion fatigue. A range of scratching and straining electrode techniques have been employed. While it is not possible to go into detail, the results have to be examined in relation to the techniques employed, e.g., has repassivation started before the scratching or straining has stopped It is important also to know whether the current measured under potentiostatic conditions is a complete anode current or the difference between an anode current and a cathode current (most commonly due to H" " ion reduction). Typical repassivation rates correspond to an equation (2 1 ) of the type ... 

In crevice corrosion, a net cathodic reaction occurs on surfaces outside the crevice, whereas a net anodic reaction occurs on the surfaces inside the crevice. The metal provides the electron path and the electrolyte within the occluded region and in the bulk provides the ionic path to complete the circuit. The types of electrochemical reactions are no different from those that occur in other forms of corrosion. 

Bimetallic corrosion and other forms of corrosion continued to cause service failures. In 1962, a report was sent to the British Ministry of Defense stating that a copper alloy end plate had fallen off a seawater evaporator in a submarine because the steel bolts with which it was secured had effectively dissolved through galvanic action. In 1982, the nose wheels failed on two Royal Navy Sea Harriers that had returned from the Falklands War. Studies showed that the galvanic action was responsible for the corrosion that occurred between the magnesium wheel alloy and its stainless steel bearing. 

Atmospheric corrosion of metals is differentiated from the other forms of corrosion due to exposure of metals to different atmospheres rather than immersion in electrolytes. The spontaneous atmospheric corrosion of materials is controlled by the temperature, the relative humidity, the time of wetness, the pH of the electrolyte, and the presence of contaminants such as chlorides, NH3, SO2, NO2, and acidic fogs. In most cases, the rate equations have hmited validity due to different local atmospheric conditions. Metals spontaneously form a solid metal oxide film when exposed to dry atmospheres. The barrier oxide film reaches a maximum thickness of 2-5 nm [1-6]. The corrosion rate of metals exposed to a wet atmosphere is similar to that observed during immenion in aerated water in the presence of dissolved oxygen. Atmospheric corrosion rates decrease in dry atmospheres with corrosion mechanisms that are different from those in wet atmospheres. 

These three assumptions lead to uniform (general) corrosion. But this is only one of several corrosion forms that occur under different conditions. The other forms of corrosion depend on the deviations from the mentioned assumptions. Such deviations... 

The existence of corrosion forms typical of the oil and gas industry does not mean that this industry and the related infrastructures are not potentially subject to other forms of corrosion, such as general corrosion, galvanic corrosion, corrosion under deposit, crevice corrosion, intergranular corrosion, stress corrosion, corrosion fatigue, fretting corrosion, corrosion of welds, creep and high-temperature corrosion, lUifoim corrosion, and cathodic disbondment. 

Researchers have identified many different forms of corrosion. The rusting of automobile bodies is an example of uniform corrosion and is one of the most visible forms of corrosion. What conditions are necessary for this process to occur Another important form of corrosion is galvanic corrosion, which occurs only when two different metals contact each other in the presence of an appropriate electrolyte. What is so special about the contact of two different metals Other forms of corrosion tend to require specific conditions, yet many of these situations... 

Of interest in this connection is the fact that atmospheric corrosion mainly occurs in the form of a more or less uniformly distributed scar-shaped erosion. However, other forms of corrosions are also encountered. Thus, for instance, the impact of a humid atmosphere on zinc alloys may produce intercrystalline corrosion, and the impact of a humid atmosphere containing NH3 may lead to stress cracks in brass components. 

G 44 can be used for both stressed and unstressed specimens. Historically, it has been used for stress corrosion cracking testing, but often is used for other forms of corrosion, such as uniform, pitting, intergranular, and galvanic. It uses a 1-h cycle that includes a 10-min period in a aqueous solution of 3.5 % sodium chloride (NaCl) followed by a 50-min period out of the solution, during which the specimens are allowed to dry. This 1-h cycle is continued 24 h/day for the total number of days for the particular alloy being tested. 

The linear polarization-resistance (LPR) technique is the only corrosion monitoring method that allows corrosion rates to be measured in real time. Although limited to elec-trol3rtically conducting liquids, the response time and data quality of this technique make it superior, where applicable, to all other forms of corrosion monitoring. 

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