On the corrosion behaviour of a ferritic 18 Cr‐2 Mo‐steel — Part I

Abstract

1 Introduction Some interesting new developments have taken place in recent years in the field of ferritic stainless steel (1–16). As a material for chemical apparatus, the common highly alloyed chromium steels as listed in national standards (e.g. in the German Standard DIN 17440) have only found limited applications. The reasons are sensitivity of several of these chromium alloyed stainless steels to intergranular corrosion (especially after welding), lower corrosion resistance compared to austenitic stainless steels, and difficulties in fabricating (especially welding). It has been shown (1–16) that the intrinsic drawbacks of customary ferritics can be overcome by metallurgical measures, primarily keeping the amount of carbon and nitrogen extremely low. The solubility in the ferrite for these two elements in rather low, both occupying interstitial sites. Stainless steels of the type dealt with in this paper are therefore sometimes termed Extra Low Interstitial (ELI)‐ferritic stainless steels. At sufficiently low concentrations of carbon and nitrogen (and some other elements), the sensitivity of ferritic stainless steels to intergranular corrosion is definitely lowered, and their ductility at ambient temperature is increased, i.e. the transition temperature is lowered. An advantage of these steels is their resistance to stress corrosion cracking. They have, so far, shown no sensitivity against chloride stress corrosion cracking under realistic operating conditions. For this reason, cooling water systems using river water with a high chloride content represent a suitable field of application for these steels. They can be welded up to a wall thickness of 3mm without sensitisation and undue loss of impact strength so that tubes for heat exchangers can be made of these steels. Their development has led to alloys ranging from 18 Cr‐2 Mo‐0 Ni to about 28 Cr‐2 Mo‐4 Ni. The present paper will only deal with the 18 Cr‐2 Mo steel because this material can be compared in price and properties with the standard 18 Cr‐9 Ni‐2 Mo austenitic stainless steel. In addition, the material in question has now become available in the form of pipe and sheet.