Specific Corrosive Environments

If there is concern about corrosion, then both the corrosive medium and the temperature are important. Also, if the part is in contact with another metallic component of a dissimilar material, then galvanically assisted corrosion, which accelerates failure, is very possible. 

For more hostile environments, including marine and aerospace 

For stress-assisted corrosive conditions, consider shot peening followed by corrosion protection appropriate to environmental conditions, but only those processes applied at near-ambient temperature. 

---

Stress corrosion cracking can be avoided by selecting materials that are not susceptible in the specific corrosion environment or, less certainly, by stress relieving by annealing after fabrication and welding. 

The term SCC can be defined as the initiation and propagation of cracks in a metallic component as a result of the combined influence of a mechanical stress and a specific corrosive environment . The stresses involved may be externally applied working stresses or internal residual stresses (often produced by deformation... 

Cracking is more likely to occur on exposure to heat (eg, welding or high-temperature service) or to specific corrosive environments. See Figure 6-11. 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

Stress Corrosion Cracking One of the serious forms of pipeline corrosion is see. This form of corrosion consists of brittle fracture of a normally ductile metal by the conjoint action of a specific corrosive environment and a tensile stress. In the case of underground pipelines, see affects the external surface of the pipe, which is exposed to soil/ground water at locations where the coating is disbonded. 

Figure 20.69 shows how a given anode can exhibit either active or passive behavior depending on the specific corrosive environment. 

Some specific corrosion environments, in the presence of applied tensile stress on the metal siuface (above some threshold vtilue), can cause stress-corrosion cracking (SCC). The somce of stresses can be extemtil, but residual stresses can tilso cause SCC failures. Specific corrosive pollutants, which may contribute to the SCC of carbon steels, are for example, the carbonates in water. Sulfide stress cracking (SSC) commonly occms on the outside of a pipe where sulfate-reducing bacteria me present at a soil pH of 3-7. Because of the produced iron sulfide (FeS), which adsorbs readily on the steel surface, hydrogen atoms generated... 

Mechanical forces (e.g., tensile or compressive forces) will usually have minimal effects on the general corrosion of metals and compressive stresses even reduce the susceptibility of metals to crack. In fact, shot peening is often used to reduce the susceptibility of metallic materials to fatigue, stress corrosion cracking (SCC), and other forms of cracking. However, a combination of tensile stresses and a specific corrosive environment is one of the most important causes of catastrophic cracking of metal structures. 

Stress-corrosion cracking (SCC) is a fracture, or cracking, phenomenon caiised by the combined action of tensile stress, a susceptible alloy, and a specific corrosive environment, llie metal may show little evidence of general corrosion attack, although sl ht localized attack in the form of pitting or crevice corrosion may be visible. Usually, only specific combinations of metallurgical and environmental condi-... 

Corrosion also occurs as a result of the conjoint action of physical processes and chemical or electrochemical reactions (1 3). The specific manifestation of corrosion is deterrnined by the physical processes involved. Environmentally induced cracking (EIC) is the failure of a metal in a corrosive environment and under a mechanical stress. The observed cracking and subsequent failure would not occur from either the mechanical stress or the corrosive environment alone. Specific chemical agents cause particular metals to undergo EIC, and mechanical failure occurs below the normal strength (5aeld stress) of the metal. Examples are the failure of brasses in ammonia environments and stainless steels in chloride or caustic environments. 

Packed vs Plate Columns. Relative to plate towers, packed towers are more useful for multipurpose distillations, usually in small (under 0.5 m) towers or for the following specific appHcations severe corrosion environment where some corrosion-resistant materials, such as plastics, ceramics, and certain metaUics, can easily be fabricated into packing but may be difficult to fabricate into plates vacuum operation where a low pressure drop per theoretical plate is a critical requirement high (eg, above 49,000 kg/(hm ) (- 10, 000 lb/(hft )) Hquid rates foaming systems or debottlenecking plate towers having plate spacings that are relatively close, under 0.3 m. 

Substituting one alloy for another may be the only viable solution to a specific corrosion problem. However, caution should be exercised this is especially true in a cooling water environment containing deposits. Concentration cell corrosion is insidious. Corrosion-resistant materials in oxidizing environments such as stainless steels can be severely pitted when surfaces are shielded by deposits. Each deposit is unique, and nature can be perverse. Thus, replacement materials ideally should be tested in the specific service environment before substitution is accepted. 

The corrosive environments which cause SCC in any material are fairly specific, and the more common combinations are listed in Table 53.2. In the case of chloride stress corrosion cracking of the 530 series austenitic stainless steels it is generally considered that the risk is... 

The term corrosion fatigue is used to describe the premature failure of materials in corrosive environments caused by cyclic stresses. Even mildly corrosive conditions can markedly reduce the fatigue life of a component. Unlike stress corrosion cracking, corrosion fatigue can occur in any corrosive environment and does not depend on a specific combination of corrosive substance and metal. Materials with a high resistance to corrosion must be specified for critical components subjected to cyclic stresses. 

Aromatic azoles are specific corrosion inhibitors for copper. They also show promise for acting synergistically with other inhibitors to lower corrosion rates on mild steel. One of the more cost effective aromatic azole inhibitors is tolytriazole (TTA). It and other azoles, however, are, subject to degradation in environments containing chlorine (Roti 1985). Azoles can also be quite dangerous to human health, and must be used with care. 

A single layer of bright nickel of thickness 5 to 10 //m will supply good durability in less corrosive environments but for more exacting conditions thicknesses of 15 to 20 /urn are necessary. An example of the latter is exterior trim for motor vehicles, where specifications demand a double-layer system of nickel. 

Each practice indicates its limitations, and these are important to keep in mind. As with all accelerated tests, what is actually tested is the resistance of these alloys to IGA in the specific test environment. One must be very careful to be sure that these results correlate with longer term exposures to the field environment of interest. In addition, these tests are only useful for evaluating the susceptibility of these alloys to IGA, not to pitting or general corrosion or SCC. 

---