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Common Types of Metal Corrosion Passive Alloys and Chloride Induced Stress Corrosion To avoid galvanic corrosion to steel, it is common practice to employ stainless steel or other passive alloys. Stainless steel contains at least 10.5% chromium, which passivates the surface with a very thin chrome -oxide film. This, in turn, serves to protect against acids and other inducers of galvanic corrosion. The most practical limitations occur in environments where this chrome -oxide film can be broken down. Very typically this occurs in the presence of the chloride ion, particularly in the vicinity of areas such as welds, where tensile stress is present. Although the mechanism can be complex, the corrosion is accompanied by a distinctive destruction of grain boundaries, which characterize the morphology or metallurgical structure of the stainless steel. This is ordinarily manifested as pitting, crevice corrosion, or corrosion stress -cracking, which may proceed rapidly once initiated. Chlorides can often be present at exceptionally high levels, especially in applications such as flue desulfurization, where there is a net evaporation of water as well as leaching of coal ash. Thus, even though stainless steel will display quite good acid resistance, the corrosion can be severe due to chlorides. Chlorides tend to be quite prevalent in industrial environments, even in places where they might not be obvious, so it is always important to be wary in the use of stainless steels. Another corrosive limitation to stainless steel relates to oxygen depletion. Since the passivity of stainless steel depends on a thin protective chrome oxide film, it is important to keep the surface in an oxidized state. The passive film may no longer be preserved in certain reducing environments, or where the surface is insulated from oxygen by scale or other strongly adhering deposits. The class of stainless steel most commonly considered in corrosive environments is known as austenite, but the other types (martensetic and ferritic) are also common. Over the years, many grades have been developed to improve resistance to chloride and to afford better strength, heat resistance, and welding properties to minimize the effects of stress induced corrosion. Characteristically, increased nickel content alloys are favored for high chloride applications, such as type 317L stainless steel, HastelloyTm, Inconel, or the various Haynes series alloys, such as C-276. Since these alloys are expensive, applications often involve cladding or thin "wallpapering" procedures. The use of these selections involves a great deal of welding, which must be done with a high degree of expertise, expense, and high level inspections with attention to detail, since welds are especially susceptible to stress corrosion. Sulfide Stress Cracking Somewhat akin to chloride -induced stress corrosion is sulfide stress corrosion cracking. This is common in oilfield and other applications, such as geothermal energy recovery and waste treatment. Carbon steel as well as otheralloys can reactwith hydrogen sulfide (1-12S), which is prevalent in sour oil, gas, and gas condensate deposits. Reaction products include sulfides and atomic hydrogen which forms by a cathodic reaction and diffuses into the metal matrix. The hydrogen can also react with carbon in the steel to form methane, which leads to embrittlement and cracking of the metal. CO2 Corrosion Carbon dioxide can be quite corrosive to steel (at times in excess of thousands of mils per year) due to the formation of weak carbonic acid as well as cathodic depolarization. This type of corrosion is especially devastating in oil and gas production and is apt to receive even more attention in the future due to increased use of CO2 for enhanced oil recovery. Additionally, various underground sequestering processes are being inspired by concerns over global warming. Turbulence, or gas velocity, can be a big factor in the CO2 induced corrosion of steel due to the formation and/ or removal of protective ion carbonate scale. On the other hand, FRP is not affected by these mechanisms of corrosion. Other Types of Stress Corrosion Sometimes internal stress corrosion -cracking of steels may occur unexpectedly due to mechanisms which are not yet completely understood. For example, there is some evidence this occurs with ethanol in high concentrations, especially around welds. Likewise, anhydrous methanol can be corrosive to aluminum as well as titanium.