Prevention of Corrosion in Austenitic Stainless Steel through a Predictive Numerical Model Simulating Grain Boundary Chromium Depletion

Prevention of Corrosion in Austenitic Stainless Steel through a Predictive Numerical Model Simulating Grain Boundary Chromium Depletion

M.K. Samal (Bhabha Atomic Research Centre, India)
Copyright: © 2017 |Pages: 16
DOI: 10.4018/978-1-5225-0588-4.ch013
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In this chapter, a mathematical model for rate of formation of chromium carbides near the grain boundary, which is a pre-cursor to chromium depletion and corresponding sensitization behavior in stainless steels, is presented. This model along with the diffusion equation for chromium in the grain has been used to obtain chromium depletion profiles at various time and temperature conditions. Finite difference method has been used to solve the above equations in the spherical co-ordinate system and the results of time-temperature-sensitization diagrams of four different types of alloys have been compared with those of experiment from literature. For the problem of low temperature sensitization and corresponding inter-granular corrosion in austenitic stainless steel, it is very difficult to carry out experiment at higher temperatures and justify its validity at lower operating temperatures by extrapolation. The development of predictive models is highly useful in order to design the structures for prevention of corrosion of the material in aggressive environments.
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In polycrystalline materials, corrosive attack of grain boundaries with little or no attack of the grains is called inter-granular corrosion. In practical application, the loss of cross section thickness and the introduction of cracks due to presence of inter-granular corrosion can have severe consequences for applications like pressure retaining containments. Extensive corrosion attack at the grain boundaries can result in grains dropping or falling out of the metal surface resulting in the disintegration of the material (Verlinden et al., 2007). With austenitic stainless steels, inter-granular attack is usually the result of chromium carbide precipitation (Cr23C6 type) at grain boundaries, which produces a narrow zone of chromium depletion at the grain boundary.

Austenitic stainless steels are used in a wide range of applications due to their excellent corrosion resistance and good manufacturability property. However, during the process of welding or heat treatment (during solution annealing, stress-relieving etc.), there can be carbide precipitation at the grain boundaries leading to chromium depleted zones. This process is aggravated when the material is extensively heated or slowly cooled in the temperature range of 500 to 850 deg. C (Verlinden et al., 2007; Dayal et al., 2005). When chromium level near the grain boundaries falls below 12%, the material is said to be ‘sensitized’ as there is not enough chromium in that region to form a passive layer to prevent attack of corrosive media. Hence, as the sensitized material is exposed to corrosive media during service, inter-granular corrosion takes place.

Chromium is the primary alloying element that makes stainless steel corrosion resistant. The chromium-depleted regions are susceptible to preferential corrosion attack. It has been known over the years that inter-granular stress corrosion cracking in austenitic stainless steels occurs because the chromium content immediately adjacent to the carbide near the grain boundary may be below that required for the stainless steel alloy to actively prevent corrosion by forming an adherent oxide layer to the surface of the material. If the carbides form a continuous network on the grain boundary, then corrosion can produce a separation or gap at the boundary, possible grain dropping or loss of material, formation of grain boundary cracks etc.

The chromium carbides tend to precipitate at the grain boundaries of austenitic stainless steels in the 500 to 850°C temperature range. Any exposure or thermal excursion into this temperature range during metal manufacture, fabrication, or service could potentially sensitize the steel. Common manufacturing processes such as welding, stress relief, and hot working of the metal can expose the steel to the sensitizing temperature range. The formation of chromium carbides can be reversed by solution-anneal heat treatment. A test method has been described in ASTM A262 standard. This method can be used to detect susceptibility to inter-granular attack in austenitic stainless steels. The time and temperature required to produce susceptibility to inter-granular attack is highly dependent on alloy composition, particularly the carbon content.

To avoid inter-granular stress corrosion cracking, the carbon in the alloy should be kept in solution phase (preventing chromium carbide formation). This can be achieved by rapid cooling the alloy through the sensitizing temperature range from the solution-annealed stage. The usual solution-anneal temperature in order to avoid sensitization of the material depends on the alloy and is typically carried out in the range of 1050 to 1200°C followed by rapid cooling. Resistance to chromium depletion at the grain boundary can also be achieved by reducing the carbon content to below 0.030% level (Moura et al., 2009).

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