Solutions to Corrosion Problems

Uniform corrosion, which is the most commonly encountered corrosion mechanism. It is caused by a chemical or electrochemical reaction which happens uniformly over the entire exposed surface area. Metal progressively gets thinner and eventually fails.

Corrosion which occurs in a specific location or under specific conditions. Localised corrosion is the key mode of failure of process equipment across all industries where conditions are severe and corrosion resistance is critical. This type of corrosion is commonly know as Pitting, Crevice and MIC (Microbially Influenced Corrosion).

Pitting is a very localised corrosion attack resulting in the formation of holes in the metal. Holes (pits) are generally small but they propagate rapidly leading to perforation of the metal and failure of the process equipment in a very short period of time.

A crevice attack usually occurs where there is a close gap between two surfaces. The development of concentration cells local to the two surfaces can lead to rapid progression of this type of corrosion.

Occurs as a result of the reduction in fatigue resistance of the metal in a corrosive media. This is normally encountered by premature failure of the metal under cycling conditions.

Cracking caused by simultaneous presence of tensile stress in a specific corrosive media. Chloride induced cracking of stainless steels and ammonia stress cracking of Nickel Copper alloys are examples of this type of attack. Alloys with more than 40% Nickel content are immune to chloride induced stress corrosion cracking.

Penetration of the surface of the metal by elemental Hydrogen leads to the formation of metallic hybride compounds in some materials, whereas with others it occurs by the interaction of dissolved Hydrogen atoms. This results in the propagation of fine cracks and voids in the metallic structure. Titanium alloys are particularly susceptible to Hydrogen embrittlement.

The selective attack of a metallic compound at the grain boundaries by a corrosive media. Integranular corrosion can occur in high performance Nickel based alloys if the Carbon content is not controlled and the material is sensitised (has not been correctly solution annealed leading to the the formation of intermetallic carbides at the grain boundaries). A commonly encountered form of intergranular corrosion is in the attack of stainless steels due to the formation of Chromium carbide precipitates at the grain boundaries and subsequent depletion of Chromium in the metal matrix.

Results in the electrical coupling of two dissimilar metals in a corrosive media. This leads to corrosion attack on the less noble metal. The less noble material becomes anodic while the more noble material becomes cathodic. The anodic material protects the cathodic material leading to its accelerated decay. Copper anodes on canal barges are used deliberately as sacrificial anodes to protect the hull from uniform corrosion.

Dealloying (also referred to as selective leaching) is the selective removal of one element of an alloy by a corrosion process. The corrosive media specifically attacks one component of the alloy. A common example of this is the selective removal of Zinc from Brass (dezincification).