This paper provides a checklist of aspects of welding procedures to highlight issues that require attentionto optimise the corrosion properties of welds in a range of CRAs including duplex stainless steels and some commonly used nickel alloys. Success depends on careful handling of the material to be welded, specifying the most suitable weld preparation and filler metal, controlling heat input during welding and adopting protective measures such as shielding with backing gas. Post-weld operations can also be significant in ensuring optimum performance.
It is an obvious requirement that welded joints in corrosion resistant alloys should have properties at least similar to those of the base material. However, the production of a weld implies a considerable thermal disturbance to the base material, which has been carefully processed and finally heat-treated to have the desired characteristics. Furthermore, the weld metal represents a chill casting and is therefore inhomogeneous. It is therefore a considerable achievement of alloy designers, filler metal developers, and welding engineers that welded fabrications perform well in a range of applications. However,success depends on careful handling of the material to be welded, specifying the most suitable weld preparation and filler metal, controlling heat input during welding and adopting protective measures such as shielding with backing gas. Post-weld operations can also be significant in ensuring optimum performance. Since local corrosion rather than general attack is normally the limiting factor with corrosion-resistant alloys, attention to detail is of crucial importance, as is an understanding of the restrictions that may result. This paper discusses these points individually, with some examples from relevant cases where they were critical.
HANDLING
While corrosion-resistant alloys - stainless steels and nickel alloys - do not suffer significant general corrosion, their susceptibility to localised corrosion not only depends on selection of the most suitable alloy for the application but also on attention to detail in handling and welding. By interfering with the integrity of the passive film, contamination ofthe surface of the alloy can provoke local attack. The most notable example is iron pick-up from contact with carbon steel or from tools used for both materials. Embedded particles form crevices with the base material and, in contact with some media, can initiate pitting.
The significance of contamination depends not only on the media to which the surface is exposed but also on alloy composition. In many cases the first medium that the weld will be exposed to after welding will be the water used for hydrotesting. This may be a range of qualities, from potable water to seawater, with or without chemical treatment to remove oxygen, control corrosion and inhibit bacterial activity. The temperature of the water will generally be ambient, but that can be a high temperature in hot regions of the world. Some of these grades of water, particularly at higher temperatures, can readily react with iron on the surface and form a local concentration of ferric chloride which is particularly aggressive in a crevice or pitting situation.
The sensitivity of CRA materials to ferric chloride depends upon their pitting resistance which is indicated by the Pitting Resistance Equivalent (PRE) value of the material. The PRE value is given by:
Pitting Resistance Equivalent Number = % Cr + 3.3% Mo + 16% N
Some variations to this equation with different coefficients or inclusion of other elements are sometimes used, particularly for the higher Mo content alloys. Materials with PRE values less than 40