Attack modes, corrosion

Before discussing individual materials we will introduce important terms and concepts in corrosion like attack modes, media classes, location of attack, kinetics, and their parameters. We feel that this is necessary for understanding of the discussion of the materials, because there is no generally accepted standard procedure that allows us to compare the corrosion behavior of materials as easily as other properties like mechanical strength or hardness. 

All forms of corrosion must be considered at the beginning of any test program, before discounting attack modes that are not likely to occur. Corrosion types can be divided into general (or uniform) attack and localized corrosion, in which extensive attack can occur in a very small area. Localized corrosion can be more difficult to observe. 

Studies on hot water tank enamelsin media of varying pH demonstrate a minimum corrosion rate at pH value of 4. In citric acid (pH 2), IR measurements indicate that ion exchange is the principal mode of corrosion. Distilled water (pH 7) showed evidence of a bulk dissolution mechanism with no silica enrichment of the surface layer. In neutral solutions, the first stage of attack is leaching of alkali ions, raising the pH of solution, which subsequently breaks down the glass network of the acidic oxides. 

Aluminum can be attacked by way of cathodic corrosion in strong alkaline media generated at the cathode when magnesium corrodes sacrificially in static NaCI solutions. This mode of attack destroys compatibility in alloys containing significant iron contamination. 

Metallurgically influenced corrosion is mainly composed of the corrosion due to chemical composition (alloying elements, metalloids and impurities), metallurgical properties (metallic phases, grain joints) and fabrication procedures (thermal treatments, lamination and welding). Figure 6.24 shows weld zone, dealloying, exfoliation and internal modes of attack. 

The mode of attack on metals described above is referred to as the hydrogen evolution type of corrosion, since gas is actually evolved at the more noble portion of the system. This type of corrosion requires a combination of two metals, the nobler one of w4iich has a low overvoltage, but it is not necessary that the metals should be in a massive form. For example, the addition of a small quantity of copper sulfate to an acid solution greatly expedites the rate at w hich zinc dissolves copper is deposited, by replacement, on various parts of the zinc and a large number of local short-circuited cells are set up. Another possibility, which frequently arise., is that the base metal should have in-... 

Linings to protect floors, vessels and other equipment subjected to corrosion, erosion, abrasion and/or thermal attack from chemical environments. The most common substrates for CRM linings are carbon steel and concrete, but other structural materials such as wood and plastics may also be effectively protected. CRM linings can provide any one or all of the following protection modes ... 

In the absence of water, it is presumed that no significant corrosion activity will take place. Surfaces may be moist due to precipitation, dew, hygroscopic action, and other causes. Precipitation is presumed to act in three modes by cleansing the surface of accumulated dry deposited matter, by moistening the surface, and (at low pH) by direct chemical attack upon the metal and corrosion products. 

Electrochemical corrosion is important to the stability and longevity of implants. Evidence suggests that uniform attack and crevice and pitting corrosion are the most important degradation modes with multipart orthopedic devices (17). Corrosion of devices with blood contact is more complex, due to the oxygenated flowing electrolyte. The cost of this corrosion has not been estimated, but it could be substantially greater than the battery market because the latter is a small fraction of the total cost of the device and associated medical operations. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

In order to fully appreciate the reasons for carrying out the conservation method selected, it is important to understand in the first instance how the metal or alloy was manufactured. From modern theories of corrosion of metals in marine environments, it is possible to predict the mode of corrosive attack that the artefact may have experienced while being buried or laying on the bottom of the ocean floor. Any adverse effect on the rate of corrosion on exposure to the atmosphere can possibly be predicted. From this knowledge, the most efficient methods of field treatments, storage conditions and conservation can be recommended. 

In addition to attack by reactive gases, alloys used in practical environments, particularly those involving the combustion products of fossil fuels, undergo an aggressive mode of attack associated with the formation of a salt deposit, usually a sulphate, on the metal or oxide surface. This deposit-induced accelerated oxidation is called hot corrosion. The severity of this type of attack, which can be catastrophic, has been shown to be sensitive to a number of variables including deposit composition, and amount, gas composition, temperature and temperature cycling, erosion, alloy composition, and alloy microstructure. A number of comprehensive reviews on hot-corrosion have been prepared. The purpose of this chapter is to introduce the... 

The erosion-corrosion attack of a specimen can be followed by measuring the change in weight as the reaction proceeds. Alternatively, dimensional changes can be measured. Specimens exposed using acceleration tubes are usually flat discs (sirr-faces other than the reaction surface can be protected by aluminizing). Specimens used in the rotating mode may be either round or flat in section. 

ISO 11907-2. 1995 [121] is a static method of lest used to determine the corrosivity of fire gases. The specimens (600 mg) in the form of granules or chips arc heated with an electric resistance wire (800 C) in a crucible. The effluent is contained within a closed cabinet of 20 liters volume maintained at 50 C and 65% rh. The corrosion detector consists of a resistance etched copper plated laminate, and the corrosivity is assessed as the variation in electrical resistance due to attack on the copper circuit. Two operating procedures are given, one for the condensing mode, in which the corrosion sensor is water cooled to 40 C. and one for the non-condensing mode, without water cooling to the sensor see Fig. 22. 

Cracking was found to be the most frequent failure mode. Cracking ranged from 27% to 36% of the corrosion failure mode. General corrosion was the next most frequent (17-26%) mode followed by 12-20% of localized attack. In the case of localized attack mechanisms, pitting was the most frequent failure mode followed by intergranular corrosion. The study found that steel and stainless steel were involved in the majority (48-61%) of the SCC failures reported. 

The main mode of attack is pitting of Cu-Ni 90/10 tubes. Impingement because of air bubbles on the tube surface will lead to the destruction of the protective film. Copper oxide ringlets were observed around the pits indicating corrosion occurred in an environment consisting of corrosive ions, moisture, and oxygen as shown in Figure 5.36. 

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