Super Duplex Stainless Steel Corrosion

Super duplex stainless steels are metals with an attractive combination of high strength and great resistance to stress corrosion as well as great ease of welding (Giel, 2007). As a result of these properties they have widespread application in a number of industries including oil and gas, power, pulp and paper, and petrochemical industries (Sagasegawa et al. 2008). It is currently estimated that SDSS makes up to 10% of the total applications of stainless steel in the world and that proportion is projected to rise in the future (Nilsson, 2008). However, it is now evident that SDSS is vulnerable to corrosion in certain conditions; Newton and Hausler (2005) indicate that when SDSS is exposed to environments rich in hydrogen, they face a great chance of being made brittle and susceptible to corrosion by the hydrogen. This is exemplified by recent reports of failures of parts of equipment made of SDSS on one of the North Sea oil platforms owned by BP and a sub-sea structure belonging to Shell (Newton & Hausler, 2005; Pohl & Ibach, 2007).

While most forms of corrosion in SDSS can be clearly observed, some forms don’t involve clear observable visual changes or material loss (Garfias-Mesias, 2010). According to Bradford (2008) many failures have been observed in seemingly alright stainless steel, in most such cases the failure can be attributed to internal corrosion cracking or brittleness resulting from hydrogen. Other dangerous forms of corrosion have been indicated as pitting and crevice corrosion both of which cannot be easily detected (R.A.T.D, 2009). These types of corrosion in SDSS may not manifest as clear visible changes easily detectable such as rust. There are also a number of other types of corrosion that reduce the capacity of SDSS material to bear stress; these include corrosion fatigue, stress corrosion cracking and fretting (Garfias-Mesias, 2010).

The corrosion of SDSS material is mainly an electrochemical process involving at least two types of chemical reactions when the material is immersed in aqueous solution. Generally, throughout the process of corrosion, anode and cathode reactions take place simultaneously; this means that the corrosion may be controlled by slowing down the rate of reaction at either electrode (Fontana, 2007). The corrosion process according to Figueroa (2009) is a simple enough process involving deterioration of the metal surface, the anode, which usually undergoes oxidation. Offshore environment especially sea waters is considered one of the environments that favor corrosion the most (Fontana, 2007). Basically, at normal temperature conditions SDSS only form a passive film on their surface as a result of oxidation, the film is considered to be a form of hydrated oxide although its exact nature has not been established according to Bradford (2008). The film protects the material from oxidation and therefore corrosion under normal condition as it maintains a high level of passivity and resistance to corrosion (Walker, 2007). The great resistance to corrosion of SDSS material results from the fact that the protective film ensures that the metal surface can only react with constituents of the solutions in which it is in contact. However, some conditions can allow the passivity to be destroyed without permitting its restoration thus exposing the surface of the metal to the solution; this leaves electrons on the surface of the metal as they pass from the metal into the solution. These electrons are the main cause of corrosion in SDSS materials (Zucchi, 2006).

Another prominent type of corrosion that affects SDSS materials has been described by Fontana (2007) as hydrogen embrittlement. This is a form of internal corrosion resulting from build-up of hydrogen concentration in the metal that leaves cracks and blisters in it. According to Bradford (2008) this type of corrosion greatly affects SDSS material more than all other forms of corrosion.