Stress corrosion cracking inhibitors

To prompt inhibitor addition to a gas scrubbing system solution prone to cause stress-corrosion cracking of carbon steel when the potential moves towards a value at which stress-corrosion cracking is known to occur. 

Nature of the environment This is usually water, an aqueous solution or a two- (or more) component system in which water is one component. Inhibitors are, however, sometimes required for non-aqueous liquid systems. These include pure organic liquids (Al in chlorinated hydrocarbons) various oils and greases and liquid metals (Mg, Zr and Ti have been added to liquid Bi to prevent mild steel corrosion by the latter ). An unusual case of inhibition is the addition of NO to N2O4 to prevent the stress-corrosion cracking of Ti-6A1-4V fuel tanks when the N2O4 is pressurised... 

Although halide ions are aggressive in near-neutral solutions they can be used to improve the action of inhibitors in acid corrosion (see Practice Acid Solutions). Variations exist among the halides, e.g. chloride ions favour the stress-corrosion cracking of Ti in methanol whereas iodide ions have an inhibitive action ... 

Caustic embrittlement corrosion (caustic induced, stress corrosion cracking), which occurs as an intergranular form of corrosion where localized stresses and strains are present (and some silicate, which acts as a general corrosion inhibitor that protects grains at the expense of the grain boundaries). 

Water having a high level of dissolved solids, more than 1000 ppm (mg/1) and up to perhaps 20,000 + ppm (mg/1), esp. chlorides. Used sometimes as a source of makeup water, the high chlorides often vary with the seasons and interfere with or reduce the corrosion-inhibiting properties of certain chemical inhibitors, as well as increasing risk of specific corrosion processes, such as stress corrosion cracking of stainless steels. 

A further progress perspective in the field of water-based paints could be their use for painting ships [205]. However, these studies are only in the beginning. The main difficulty is corrosion prevention. Maximum protection from stress corrosion cracking is ensured by alkyd paints combined with phenolic epoxy paints during a 6.5-year exposure [206]. These data were obtained on pre-contaminated steel plates. In Russia now water-based dispersion primers with anticorrosive properties have been developed and manufactured [207]. This was possible after incorporation of corrosion inhibitors combined with surfactants into the formulations. 

Corrosion Electrochemistry Iron Dissolution Mechanisms Corrosion Inhibition and Inhibitors Stress Corrosion Cracking... 

Another possible adverse effect of inhibition is an increased rate of corrosion of a metal in the system other than the one for which the inhibitor was selected to protect. For example, some amines protect steel admirably but will severely attack copper and brass. Nitrites may attack lead and lead alloys such as solder. In some cases, the inhibitor may react in the system to produce a harmful product. An illustration of this is the reduction of nitrite inhibitors to form ammonia that causes stress corrosion cracking of copper and brass. The only way to avoid these problems is to know the metallic components of a system and be thoroughly familiar with the properties of the inhibitor to be used (Nasr-El-Din et al. 2002). 

Addition of Inhibitors. It is possible to add inhibitors for controlling two kinds of corrosion in boiler systems, namely, stress-corrosion cracking and return-line corrosion. The first can be minimized by addition of phosphates, as mentioned previously. 

In practice, inhibitors are often defined according to their field of application. In aqueous environments, inhibitors for acid environments are typically used to minimize metal corrosion during pickling of steel, an operation that removes oxide scales by dissolution in an acid. In the petroleum industry, large quantities of inhibitors for acid environments are used to avoid corrosion of drilling equipment. Inhibitors for neutral environments are used above all for the protection of cooling-water circuits. Inhibitors not only reduce the rate of uniform corrosion, but they also serve to protect metals from localized corrosion and stress corrosion cracking [18]. 

Hydrochloric acid can result in pitting and stress corrosion cracking. Carbon steels can tolerate a relatively high corrosion rate during hydrochloric acid cleaning, provided an inhibitor is added, because cleaning operations are short and other damage mechanisms are not a factor. 

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