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

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.     

Role of thiosulfate in susceptibility of AISI 316L austenitic stainless steels to pitting corrosion in 3.5% sodium chloride solutions

The susceptibility of AISI (American iron and steel institute) 316L austenitic stainless steel alloy to pitting corrosion was assessed in 3.5% chloride solutions containing various concentration of thiosulfate ions, a main sulfide oxidant product, spanning across values of 0.001, 0.005, 0.01, 0.05 and 0.1 M, at temperatures of 23, 50 and 80°C. The potentiodynamic scan results indicated that low thiosulfate concentrations promote the chloride attack and the aggressiveness of thiosulfate species depends on the chloride to thiosulfate ratio and the test temperature. Increasing temperature apparently promotes the ionic activity of Cl^– and S₂O₃^2– The thiosulfate to chloride ratio plays an essential role in pitting the intensity of the AISI 316L stainless steel alloy and was found to be dependent on the test temperature.     

Repair of Tube–Tubesheet Weld Cracks in a Cracked Gas/Steam Heat Exchanger

Serious cracks were found at tube-to-tubesheet welds during the start up operation of a cracked gas/steam heat exchanger in a gas plant. The cause of the failure has been analyzed, and the exchanger is repaired. The root cause of the cracks is mainly attributed to poor workshop in the tube–tubesheet welds. Failure analysis and repair procedure were reported in the present article. Several months of operation after the repair confirmed that the repair was successful.     

Metallurgical and environmental factors affecting the pitting behavior of UNS S 32205 duplex stainless steel in chloride solutions

The effect of temperatures up to 80 °C, dissolved CO₂ (sat.), and flow (agitation) up to 1000 rpm on the breakdown potential of mill‐annealed UNS S 32205 duplex stainless steel (DSS) has been evaluated in 1 M NaCl solution using potentiodynamic polarization technique. The results were compared with heat‐treated samples (furnace heated at 850 °C for 10 or 60 min followed by water quench). The aim of the heat treatment is to simulate sigma phase precipitation. The results show dependence of pitting potential on the various factors (sigma phase, test temperature, dissolved CO₂ (sat.), or solution flow) being studied. In general, the mill annealed or heat‐treated UNS S 32205 DSS alloys are immune to pitting at room temperature (23 °C). At 50 °C however, pitting corrosion is evident and the intensity of such attack is increased with the presence of dissolved CO₂, increasing isotherm held duration and/or seawater temperature. In contrast, agitation speed of 350 rpm (Re of 12 880) or above (sufficiently to yield turbulent flow) has shifted the pitting potential to more positive values. Lower speed of 100 rpm, however, is negligibly different from the stagnant conditions.     

Influence of cold work and sigma phase on the pitting corrosion behavior of 25 chromium super duplex stainless steel in 3.5% sodium chloride solution.

The effect of cold work (up to 16% strain) and sigma phase precipitation (at 850 °C for 10 and 60 min) on the pitting resistance of 25 chromium super duplex stainless steel were investigated in 3.5% sodium chloride solution at 70 and 90 °C. Anodic polarization scans for cold worked samples revealed immunity to pitting attack at 70 °C even with 16% strain. At 90 °C, the alloy still showed high pitting resistance, pitting occurring at about 600 mV (SCE) for the 16% plastic strain samples. A serious deterioration of the pitting corrosion resistance was found after heating the alloy at 850 °C for 10 min resulting in a clear drop in the pitting potential at 90 °C. After heating for 60 min, the material showed rapid deterioration of pitting corrosion resistance at 70 °C.     

Multiobjective optimization of microalgae (Chlorella sp.) growth in a photobioreactor using Box‐Behnken design approach

This study investigates a parameter optimization approach to maximize the specific growth rate of the Chlorella vulgaris microalgae species, its biomass productivity, and CO₂ capture rate. For this purpose, the Box‐Behnken experimental design technique is applied with temperature, nitrogen to phosphorus ratio, and light‐dark cycle per day, as the growth controlling parameters. For each response, a quadratic model is developed separately describing the algal specific growth rate, biomass productivity, and CO₂ capture rate, respectively. The maximum specific growth rate of 0.84 d^?1 is obtained at 25 °C, with a nitrogen to phosphorus ratio of 3.4:1, and light‐dark cycles of 24/0 h. Maximum biomass productivity of 147.3 mg L^?1 d^?1 is found at 30 °C, with a nitrogen to phosphorus ratio of 3:1, and light‐dark cycles of 12/12 h. In addition, the maximum CO₂ capture rate of 159.5 mg L^?1 d^?1 is also obtained at 30 °C, with a nitrogen to phosphorus ratio of 4:1, and light‐dark cycles of 23/1 h. Finally, a multi‐response optimization method is applied to maximize the specific growth rate, biomass productivity, and CO₂ capture rate, simultaneously. The optimal set of 30 °C, a nitrogen to phosphorus ratio 3:1, and light‐dark cycles 16/8 h, provide the maximum specific growth rate of 0.66 per day, biomass productivity of 147.6 mg L^?1 d^?1, and CO₂ capture rate of 141.7 mg L^?1 d^?1.

     

Stress-Corrosion Cracking and Galvanic Corrosion of Internal Bolts from a Multistage Water Injection Pump

Nineteen out of 26 bolts (studs) used for assembly of multistage water pump showed severe corrosion and cracking after brief service in a severe working environment that contained saline water, CO₂, and H₂S. The failed bolts and intact nuts were supposed to be made out of a special type of austenitic stainless steel as per ASTM A 193 B8S and ASTM A 194, respectively. However, the investigation showed that bolts and nuts are made from two different alloys: an austenitic stainless steel and a nickel-base alloy. The difference in the corrosion resistance of these two alloys led to severe galvanic corrosion. The galvanic coupling between bolts and nuts in addition to the severe working environment played major role in the premature failure of bolts. The mechanisms of bolt failure were galvanic corrosion for bolts that were in direct contact with the environment and stress-corrosion cracking in the bolts remote from the severe environment. The stress-corrosion cracking was influenced by a bad fit between the bolts and nuts threads. This resulted in a crevice and the development of an aggressive chemistry between the engaged bolt/nut threads. All factors required to cause stress-corrosion cracking were available, namely, stressed bolts (bolts under tensile stress), temperatures above 60 °C, and chloride ions.