TWI Industrial Member Report Summary 712/2000
P Woollin and B J Ginn
Background
Highly alloyed Ni-Cr-Mo alloys are used in very corrosive acid chloride media when stainless steels are inadequate. The pitting resistance in chlorides of austenitic stainless steels, which have a similar face centred cubic structure to nickel alloys but with lower levels of nickel, is primarily controlled by the levels of Cr, Mo and N, as formalised in the pitting resistance equivalent number (PREN = %Cr + 3.3%Mo + x%N, where values of x from 13 to 30 have been proposed), which gives a ranking of different compositions. Nitrogen alloying of austenitic stainless steels is an increasingly viable means of improving pitting resistance in chloride media, provided the nitrogen remains in solid solution. Addition of nitrogen to TIG weld metal via the shielding gas has been demonstrated as a successful means of improving pitting resistance associated with autogenous welding.
This project examined the extension of this concept to nickel alloys intended for very severe acid chloride media, which typically do not have significant levels of nitrogen but instead rely on Cr and Mo for pitting resistance. Preferential weld metal corrosion remains an industrial concern for these nickel alloys and the similarity of the nickel alloy and austenitic stainless steel systems suggests that nitrogen alloying could be beneficial also for nickel alloys. Autogenous TIG welds were examined in three sheet materials and nitrogen pick-up from the shielding gas was studied, with emphasis on its effect on corrosion resistance.
Objectives
- To quantify nitrogen pick-up from the shielding gas in a range of nickel alloy TIG weld metals.
- To quantify the effect of nitrogen pick-up in nickel alloy weld metal on pitting resistance in acid chloride media.