Cast irons crevice corrosion

An acceptable life for undergound pipelines can sometimes be realized through the use of corrosion-resistant piping. Copper, aluminum, and stainless steel piping are sometimes used for this purpose. All three alloys can have greater corrosion resistance than carbon steel or cast iron. However, they are not immune to corrosion and are often more susceptible to localized corrosion such as pitting, crevice corrosion, and SCC than carbon steel or cast iron. Further, corrosion protection in the form of coatings and cathodic protection are frequently used. 

As an alternative to the use of acids, sequestering agents have been employed to dissolve the corrosion products without attacking the parent metal. The most effective formulations are based on the derivative of ethylene diamine-tetra acetic acid (EDTA). Lead artefacts from the Mary Rose were cleaned in a 10% solution of this compound. The use of EDTA is not recommended for cast iron as the graphite flakes embedded within the corrosion products are also dissolved. As with the use of acids, the shape of the artefact is altered if the corrosion layers are very thick and it is also difficult to wash out all the solutions from cracks, crevices and pores in the artefact after cleaning. 

If, on the other hand, a less noble metal with free surface is connected to a more noble component with a crevice, the coupling will counteract crevice corrosion in the latter. Certainly, in many cases of stainless steel exposed to seawater, crevice corrosion is prevented by contact with unalloyed/low-alloy steel or cast iron. An example is stainless steel pumps or valves coupled to ordinary steel or iron pipes. 

The standard steels of the type SAE 316 (DIN-Mat. No. 1.4401, X5CrNiMol7-12-2) are not suitable for seawater-exposed pipes and fail as a result of pitting and crevice corrosion [155, 156]. The sensitivity to pitting corrosion of these standard steels can be further increased by deposits of maritime bacterial films [157]. Despite these facts, these steels are frequently used as materials for pump parts and have worked well as such because they are cathodically protected by contact with other parts made of less noble materials, e.g. pump casing made of cast iron [130]. [158] reports on tests of the cavitation behaviour of the pump materials GX5CrNiMol9-ll-2 (DIN-Mat. No. 1.4408) in 3% NaQ solution. 

Cast irons behave similarly to carbon steels. They show susceptibility to pitting and crevice corrosion in seawater and similar environments. Addition of nickel-[Pg.509]

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

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