CORROSION SCIENCE SECTION CORROSION—Vol. 64, No. 10 755 Submitted for publication February 2008; in revised form, May 2008. ‡ Corresponding author. E-mail: rkaul@cat.ernet.in. * Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore, 452013, India. ** Department of Metallurgical Engineering and Material Science, IIT (Bombay), Powai, Mumbai, 400076, India. *** Material Science Division, Bhabha Atomic Research Centre, Mumbai, 400085, India. **** Department of Mechanical Engineering, IIT, Kharagpur, 721 302, India. Laser Surface Treatment for Enhancing Intergranular Corrosion Resistance of AISI Type 304 Stainless Steel R. Kaul, ‡, * S. Mahajan,** V. Kain,*** P. Ganesh,* K. Chandra,*** A.K. Nath,**** and R.C. Prasad** ABSTRACT A novel laser surface modification approach to suppress sen- sitization in AISI 304 (UNS S30400) austenitic stainless steel is described. Surface modification of austenitic stainless steel was carried out with a 10-kW carbon dioxide (CO 2 ) laser in both continuous wave and pulse-modulated modes. After laser surface modification, the material was subjected to a sensi- tization heat treatment at 923 K for 9 h. The degree of sen- sitization was determined by electrochemical potentiokinetic reactivation test while the susceptibility to intergranular cor- rosion was determined using the ASTM A262, practice B test. The results of the study demonstrated that the laser-melted surface exhibited significantly higher resistance against sen- sitization and intergranular corrosion than the base metal. The laser-melted surface, even after being subjected to severe sensitization heat treatment, developed comparable or even a lower degree of sensitization than the base metal in the as- received condition. Enhanced immunity against sensitization of the laser-treated surface is attributed to its duplex micro- structure and higher fraction of low-angle grain boundaries. The highlight of the investigation was that a laser surface melting treatment of unstabilized austenitic stainless steel brings about significant reduction in its susceptibility to sensi- tization during subsequent exposure to a susceptible tempera- ture region and to intergranular corrosion during service. KEY WORDS: intergranular corrosion, laser surface treatment, sensitization, stainless steel INTRODUCTION Austenitic stainless steels are used in a variety of in- dustries mainly due to their good corrosion resis- tance, strength, and formability. 1-3 Although austenitic stainless steels have high resistance to uniform corro- sion, they are prone to localized corrosion, such as crevice, pitting, intergranular corrosion (IGC), and stress corrosion cracking (SCC). Susceptibility to lo- calized corrosion and SCC is caused mainly by the presence of chloride ions in the associated environ- ments. In nuclear fuel reprocessing, waste manage- ment industries, and in many chemical industries, the main corrosion problem is IGC when nitric acid (HNO 3 ) is used as the process fluid. 4-5 The basic cause of IGC is sensitization of stainless steels. IGC of aus- tenitic stainless steel arises from intergranular precip- itation of Cr-rich carbides M 23 C 6 during exposure to a temperature range from 773 K to 1,073 K. Intergranu- lar carbide precipitation is accompanied by the devel- opment of chromium-depleted zones adjacent to grain boundaries. Chromium-depleted zones, being anodic with respect to the grain interior, are preferentially at- tacked in the corrosive environment leading to IGC. 1-6 Sensitization is also a main reason for intergranular stress corrosion cracking (IGSCC) of stainless steel weldments in certain environments, e.g., oxidizing water chemistry in boiling water reactors. 7 There are several past studies involving the use of laser surface melting (LSM) treatment for enhanc- 0010-9312/08/000131/$5.00+$0.50/0 © 2008, NACE International