ABSTRACT
Stainless steels (SS) are the workhorse alloys of aggressive industrial applications. Stainless steels are obtained by adding chrome into iron alloys. Other elements such as molybdenum are also added to increase the resistance of these alloys to localized corrosion. Stainless steels may exist as ferritic, austenitic, martensitic and duplex materials. Localized corrosion of SS is generally promoted by chloride ions (Cl-) in localized spots of a passivated metal surface. This paper shows results comparing the localized corrosion resistance of seven martensitic, ferritic and austenitic stainless steels in deaerated 10,000 ppm Cl- solution at ambient temperature. Results show that in general the resistance to localized corrosion correlates well with the value of the pitting resistance equivalent number (PRE), regardless of their crystalline microstructure (i.e. austenitic vs ferritic).
INTRODUCTION
Stainless steels (SSs) are alloys based on the element iron (Fe) plus the addition of alloying elements such as chromium (Cr), nickel (Ni), molybdenum (Mo), copper (Cu), nitrogen (N), Manganese (Mn), and other minor elements such as niobium (Nb), titanium (Ti), Aluminum (Al), etc. 1 SSs are used almost in every industry where carbon steels or low alloy steels do not perform well. In the USA, stainless steels represent the third largest volume of sold alloys after carbon / low alloy steels and aluminum alloys. 2 SSs are less prone to corrode because of the spontaneous development of a chromium-rich passive film on the exposed surface. A minimum 11-12% weight percent of Cr is necessary to form a protective continuous layer of chromium oxide. Stainless steels may be classified per their metallurgical structure in (1) ferritic, (2) martensitic, (3) austenitic, and (4) duplex. Ferritic SSs are body-centered cubic (bcc) the same as the base element Fe while austenitic SSs are face-centered cubic (fcc) due to the incorporation Ni and/or Mn as alloying elements. Martensitic SSs are obtained by rapid quenching of the steel from the temperature of the austenitic phase and they are body-centered tetragonal (bct) or cubic. Duplex stainless steels contain similar volumetric amounts of austenite and ferrite. 3
