Rev. Téc. Ing. Univ. Zulia. Vol. 39, Nº 7, 35 - 40, 2016 doi:10.21311/001.39.7.04 Corrosion Resistance Study of Heat Treated 420 Martensitic Stainless Steel and 316 Austenitic Stainless Steel in Dilute Acid Concentrations Roland Tolulope Loto 1, 2* , Osazaenaye Aiguwurhuo 1 and Ukene Evana 1 1 Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria 2 Department of Chemical, Metallurgical & Materials Engineering, Tshwane University of Technology, Pretoria, South Africa *Corresponding author (email: tolu.loto@gmail.com) Abstract The corrosion resistance of quenched 420 martensitic and annealed 316 austenitic stainless steels were evaluated through coupon analysis, potentiodynamic polarization technique and optical microscopy in 1-6M H 2 SO 4 and HCl acid media. Results show that the heat treated 316 stainless steel had a significantly higher corrosion rate than the 420 martensitic steel. Heat treatment greatly improved the corrosion resistance and passivation characteristics of the martensitic steel at all acid concentrations studied with an average percentage improvement of 62% and 56.2% in corrosion rate from H2SO4 and HCl acid. The heat treated austenitic steel showed limited change in corrosion resistance with an average percentage improvement of 30.9% and 29.25% in corrosion rate from H2SO4 and HCl acid. Micrographs from optical microscopy showed a less corroded morphology for martensitic steel in comparison to the austenitic steel due to the presence of retained austenite and martensite formation. Key words: Corrosion; Heat Treatment; Passivation; Acid 1. INTRODUCTION The foundation of modern industry is stainless steel due to its versatile application in aqueous corrosive environments in a significant number of industries (Khatak and Baldev, 2002; Oberndorfer et al., 2002) Advance manufacturing techniques and high volume production of stainless steels has resulted in the availability of cost effective and corrosion resistant steels (Marshall, 1984). Austenitic stainless steels grades generally have strong corrosion resistance in mildly corrosive acid, industrial and marine environments (Raymond and Higgins, 1985). Martensitic stainless steels have good mechanical properties and strong resistance to surface deterioration. They are used majorly in industries such as chemical processing plants, power generation devices and equipments, aerospace, oil and gas refineries and marine applications (Brickner, 1968). These grades are however susceptible to corrosion at slightly higher concentrations and in the presence of chloride ions, most especially to pitting corrosion (Dell, 1989; Betova et al., 2002). Chromium and to a lesser extent nickel and molybdenum are the most important elements in the resistance and susceptibility of stainless steels to corrosion (Palit et al., 1993; Gaudett and Scully, 1994). Their resistance is determined by their passivation characteristics, elemental composition, heat treatment and the corrosive medium. The durability of the protective covering on stainless steels is subject to its self-healing ability (Fadare and Fadara, 2013; Kempester, 1984). The film collapses when the rate of corrosion is much faster than the reforming rate leading to severe localized corrosion attack and eventually catastrophic failures. Most research conclusions state that Cl- ions diffuses through the passive films causing its breakdown at the metal/film interface, (Dong and Zhou, 2000; Strehblow et al, 1995). Stimming (1986) showed in his work that the presence of hydrogen atoms within a passive film destroys the durability of the film, promotes its breakdown and hinders the repassivation process. The need for superior corrosion resistant properties in specific industrial applications necessitates performance improvement on the surface property and metallurgy of these steels (Millano et al., 2006; Aponte et al., 2008). Altering the microstructure of stainless steel significantly improves their corrosion resistance. Microstructural constituents such as grain size, phases, precipitates, flaws and inclusions are strongly modified by heat treatment to effect changes in their mechanical, chemical and surface properties based on the austenite/martensite formation, changes in grain size and defects (Rajan, 1998). Heat treatment through quenching involves cooling a metal rapidly to produce a martensite transformation (harder metal) after heating the metal above the upper critical temperature. Rapid cooling causes a part of the austenite to change to martensite. The hardness of a metal due to quenching is a product on its chemical constituent and quenching process. Previous researches on the effect of heat treatment processes on the corrosion resistance of stainless steel have given mixed results. Lu et al (2015) studied the changes due to heat treatment on the microstructure characteristics and the electrochemical behaviour of plastic mold steel in chloride solution. Results showed that the corrosion resistance of the as-quenched martensitic steels increased with austenitizing temperature but decreased after tempering. Nasery et al (2011) studied the effects of astenitizing temperature, tempering temperature and time, and on the microstructure, mechanical and corrosion properties of AISI 420 martensitic steel. Results showed that the temperature significantly influenced the mechanical properties of the steel. The