S. O. Nisar

Bio: S. O. Nisar is an academic researcher from University of Bahrain. The author has contributed to research in topics: Surface roughness & Austenitic stainless steel. The author has an hindex of 1, co-authored 1 publications receiving 8 citations.

Effect of Surface Finish on the Pitting Corrosion Behavior of Sensitized AISI 304 Austenitic Stainless Steel Alloys in 3.5% NaCl Solutions

Abstract: The effect of surface condition down to 120 and 1000-grit finish, corresponding to 1.47 and 0.06 microns RMS (Root Mean Square), respectively, on the pitting corrosion behavior of sensitized and mill-annealed AISI (American iron and steel institute) 304 stainless steel was studied in 3.5% NaCl solutions at 23 and 50°C by electrochemical methods. The polarization curves have revealed a clear dependence of pitting corrosion on the surface finish, on the degree of sensitization, as well as on the test temperature. Surface condition has made a significant contribution to pit initiation in that the pitting potential was lowered as the surface roughness increased. The deleterious effect of surface roughness on the pitting potential of the AISI 304 stainless steel alloy in 3.5% NaCl is more pronounced on sensitized samples and becomes more evident with increasing sensitization time and test temperature.

Microstructure and Properties of Surface-Modified Plates and Their Welded Joints.

Abstract: The surface of Q235 low carbon steel was modified by the metal inert-gas welding (MIG) method; a 304 stainless steel surfacing layer was fabricated to improve the properties of Q235 low carbon steel. For practical industry application, keyhole tungsten inter gas (K-TIG) welding was used to weld the surface-modified plates. The microstructure, elemental distribution, micro-hardness, and corrosion resistance of the surface-modified plates and the welded joints were analyzed. The corrosion tests of welded joints and surface-modified plates were carried out with the electrochemical method and hydrochloric acid immersion method, respectively, and surface morphology after corrosion was studied. The results show that the surface-modified plates and their welded joints were defect-free. The microstructure of the surfacing layer consisted of austenite, martensite, and ferrite; and the microstructure of the weld consisted mainly of martensite. The hardness and corrosion resistance of the surfacing layer was superior to that that of low carbon steel. The micro-hardness of the weld is higher than that of the stainless steel surfacing layer and the base material. The corrosion resistance of the surfacing layer is the best, and the corrosion resistance of the welding seam is better than that of the base material.

Influence of medium on corrosion and microstructural properties of HTCS-130 tool steel for hot work

Abstract: In this paper the corrosion resistance of tool steel for hot work in the industrial emulsion medium (Lenox Band-Ade semi-synthetic oil + water, in ratio 1:10), water and 3.5% NaCl medium was tested. By Tafel’s extrapolation from polarization curves it was established that the tested tool steel showed extremely high corrosion rate in water and 3.5% NaCl medium in contrast to medium of Lenox BandAde emulsion. The double higher corrosion rate of tool steel is recorded in the chloride medium as opposed to that obtained in water, indicating that the tested steel is more corrosion resistant in water. The obtained results were confirmed by the method of electrochemical impedance spectroscopy. The sample of tool steel in the Lenox Band-Ade emulsion medium showed far greater value of charge transfer resistance Rct than that obtained in the water medium and 3.5% NaCl, which means that the Lenox Band-Ade emulsion formed a thicker oxide layer that has the role of barrier in further penetration of aggressive ions from the solution. The conducted SEM analysis after electrochemical measurements in 3.5% NaCl medium indicate the occurrence of pitting corrosion caused by breaking the passive surface of the material as a result of the action of aggressive ions from the solution. The EDS analysis of formed pits has been shown the increased oxygen content, but also the higher presence of sodium and chlorine, which accumulate in the pits, and come from the medium. Molybdenum and tungsten showed the highest presence in white deposits on the sample surface, which represent parts that did not completely cover with oxide layer, which is why the smallest oxygen content was recorded at these sites. The obtained corrosion rate values in the water medium and the chloride medium indicate that the studied tool steel is poorly stable in water and chloride media, and because of that it can only be used in exceptional cases under conditions in which it will come into contact with water or chloride medium.

Pitting behavior of friction stir repair-welded 304L stainless steel in 3.5% NaCl solution at room temperature: role of grain and defect structures

Abstract: Stainless steel (SS) canisters used for storing spent nuclear waste are prone to chloride-induced stress corrosion cracking. Friction stir welding (FSW), being a low heat input process, can be used for repair welding of these canisters. In this study, an artificial crack was introduced on the 304L SS coupons by an electric discharge machining process. The artificial crack was repair-welded by FSW by maintaining the tool temperature a constant at two levels (725 and 825 °C). Friction stirring significantly reduced the grain size of the stirred zone (SZ) from about 47 to 2–4 μm. The fraction of low-angle grain boundaries increased in the SZ from 2 to 37–43%. On the other hand, the fraction of special grain boundaries (Σ3 and Σ9) that was ~ 50% in the base metal reduced to < 10% in the SZ. During friction stirring, the oxide layer of artificial crack was broken and aligned to form a spiral defect called lazy-S structure. All these microstructural changes affected the corrosion behavior of the FSW specimens when tested in 3.5% NaCl at room temperature using cyclic polarization, chronoamperometry, and electrochemical impedance spectroscopy. The FSW specimens showed lower polarization resistance and lower transpassive potential than those of the base metal specimens. However, the pitting protection potentials of the FSW specimens were higher than that of the base metal. The pitting behavior of the FSW specimens was influenced more by the preferential attack on the lazy-S region than by passive film breakdown. The flat band potentials of the passive film formed on the FSW specimens were more positive than that of base metal. The charge carrier density of passive film formed on the FSW specimens was higher than that of the base metal. The higher fraction of low-angle grain boundaries present in the FSW specimens could supply more number of misorientation dislocations at the metal/film interface which could form anion vacancies by a climb process leading to formation of oxide at these locations without stressing the substrate. Therefore, low-angle boundaries are considered helpful for formation of stress-free passive films that will be highly stable and enhance both pitting and stress corrosion cracking resistance.