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Corrosion of Titanium Alloys in High Temperature Seawater
Materials of choice for offshore structures and the marine industry have been increasingly favoring materials that offer high strength-to-weight ratios. One of the most promising families of light-weight materials is titanium alloys, but these do have two potential Achilles’ heels: (i) the passive film may not form or may be unstable in low oxygen environments, leading to rapid corrosion; and (ii) titanium is a strong hydride former, making it vulnerable to hydrogen embrittlement (cracking) at high temperatures in low oxygen environments. Unfortunately, such environments exist at deep sea well-heads; temperatures can exceed 120 °C, and oxygen levels can drop below 1 ppm. The present study demonstrates the results of investigations into the corrosion behavior of a range of titanium alloys, including newly developed alloys containing rare earth additions for refined microstructure and added strength, in artificial seawater over the temperature range of 25 °C to 200 °C. Tests include potentiodynamic polarization, crevice corrosion, and U-bend stress corrosion cracking.
KEYWORDs : titanium, seawater, adhesive, high temperature, corrosion
AUTHORs : J. J Pang and D. J. Blackwood†

Page : 190 - 194 DOWNLOAD
Corrosion Behaviour of DH36 Steel Used for Oil Platform in Splash Zones
The splash zone is the most corrosive area of the marine environment, and the corrosion of steel structures exposed in this area is a serious concern. DH36 steel is one of most commonly used steels for offshore oil platforms in China, and its corrosion behaviour in splash zones was studied in this paper. Polarization curves were obtained from the corroded steel exposed in this area while the morphologies and rusts of the rust steel were characterized using scanning electron microscopy and X-ray diffraction. Double rust layers were formed in the splash zone. The inner layer contained magnetite and fine flaky lepidocrocite, and the outer layer was composed of accumulated flaky lepidocrocite and a small amount of goethite. In
the wet period, the iron dissolved and reacted with lepidocrocite, and magnetite appeared, while the magnetite was oxidized to lepidocrocite again during the dry period. Electrochemical reduction and chemical oxidization cycled in intermittent wetting and drying periods, and magnetite and lepidocrocite were involved in the
reduction reaction, leading to serious corrosion.
KEYWORDs : steel, corrosion, rust, splash zone
AUTHORs : J. G. Liu†, Y. T. Li, and B. R. Hou

Page : 184 - 189 DOWNLOAD
Corrosion and Repassivation Behavior of Stainless steels in Chloride and Thiosulfate Containing Environments
In this study, the combined effect of chloride and thiosulfate ions and the effect of the ratio of the two ions on passivation in 304L, 316L, and the duplex stainless steels 2101 and 2205 are investigated using potentiostatic scratch tests. Cyclic polarization and the scratch tests were used to understand the role of anions on localized corrosion in these systems. It was found that the thiosulfate pitting began at a lower potential for 2101 than 304L in 0.6 M NaCl + 0.03 M Na2S2O3 solution. The pit morphologies for 304L, 316L, and 2101 in an 0.6 M NaCl + 0.03 M Na2S2O3 solution were very different from each other. The results indicate that the pitting switches from predominately thiosulfate pitting to chloride pitting at approximately
0.1 V.
KEYWORDs : stainless steel, chloride, thiosulfate, pitting, repassivation, pit morphology
AUTHORs : Y. S. Wang† and P. M. Singh

Page : 177 - 183 DOWNLOAD
Effect of RuCl3 Concentration on the Lifespan of Insoluble Anode for Cathodic Protection on PCCP
Prestressed Concrete steel Cylinder Pipe (PCCP) is extensively used as seawater pipes for cooling in nuclear power plants. The internal surface of PCCP is exposed to seawater, while the external surface is in direct contact with underground soil. Therefore, materials and strategies that would reduce the corrosion of its cylindrical steel body and external steel wiring need to be employed. To prevent against the failure of PCCP, operators provided a cathodic protection to the pre-stressing wires. The efficiency of cathodic protection is governed by the anodic performance of the system. A mixed metal oxide (MMO) electrode was developed to meet criteria of low over potential and high corrosion resistance. Increasing coating cycles improved
the performance of the anode, but cycling should be minimized due to high materials cost. In this work, the effects of RuCl3 concentration on the electrochemical properties and lifespan of MMO anode were evaluated. With increasing concentration of RuCl3, the oxygen evolution potential lowered and polarization resistance were also reduced but demonstrated an increase in passive current density and oxygen evolution current density. To improve the electrochemical properties of the MMO anode, RuCl3 concentration was increased. As a result, the number of required coating cycles were reduced substantially and the MMO anode achieved an excellent lifespan of over 80 years. Thus, we concluded that the relationship between RuCl3 concentration and coating cycles can be summarized as follows: No. of coating cycle = 0.48*[RuCl3 concentration, M]-0.97.
KEYWORDs : cathodic protection, insoluble anode, MMO (Mixed Metal Oxide), lifespan, ruthenium chloride, PCCP (Prestressed Concrete steel Cylinder Pipe)
AUTHORs : H. W. Cho, H. Y. Chang, B. T. Lim2, H. B. Park, and Y. S. Kim†

Page : 172 - 176 DOWNLOAD
Corrosion Fatigue of Austenitic Stainless Steel in Different Hot Chloride Solutions
Austenitic stainless steel was investigated under cyclic loading in electrolytes with different chloride contents and pH and at different temperatures. The testing solutions were 13.2 % NaCl (80,000 ppm Cl-) at 80 °C and 43 % CaCl2 (275,000 ppm Cl-) at 120 °C. In addition to S−N curves in inert and corrosive media, the fracture surfaces were investigated with a scanning electron microscope (SEM) to analyse the type of attack. The experimental results showed that a sharp decrease in corrosion fatigue properties can be correlated with the occurrence of stress corrosion cracking. The correlation of occurring types of damage in different
corrosion systems is described.
KEYWORDs : corrosion fatigue cracking, stress corrosion cracking, austenitic stainless steel, pitting, chloride
AUTHORs : A. Visser†, G. Mori, M. Panzenböck, and R. Pippan

Page : 166 - 171 DOWNLOAD
Influence of Citric Acid on the Metal Release of Stainless Steels
Knowledge of how metal releases from the stainless steels used in food processing applications and cooking utensils is essential within the framework of human health risk assessment. A new European standard test protocol for testing metal release in food contact materials made from metals and alloys has recently been published by the Council of Europe. The major difference from earlier test protocols is the use of citric acid as the worst-case food simulant. The objectives of this study were to assess the effect of citric acid at acidic, neutral, and alkaline solution pH on the extent of metal release for stainless steel grades AISI 304 and 316, commonly used as food contact materials. Both grades released lower amounts of metals than the specific release limits when they were tested according to test guidelines. The released amounts of metals were assessed by means of graphite furnace atomic absorption spectroscopy, and changes in the outermost surface composition were determined using X-ray photoelectron spectroscopy. The results demonstrate that both the pH and the complexation capacity of the solutions affected the extent of metal release from stainless steel and are discussed from a mechanistic perspective. The outermost surface oxide was significantly enriched in chromium upon exposure to citric acid, indicating rapid passivation by the acid. This study
elucidates the effect of several possible mechanisms, including complex ion- and ligand-induced metal release, that govern the process of metal release from stainless steel under passive conditions in solutions that contain citric acid.
KEYWORDs : metal release, stainless steel, citric acid, food, surface oxide
AUTHORs : N. Mazinanian†, I. Odnevall Wallinder, and Y. S. Hedberg

Page : 161 - 165 DOWNLOAD
Stress Analysis in Polymeric Coating Layer Deposited on Rigid Substrate
This paper presents an analysis of thermal stress induced along the interface between a polymeric coating layer and a steel substrate as a result of uniform temperature change. The epoxy layer is assumed to be a linear viscoelastic material and to be theromorheologically simple. The viscoelastic boundary element method is employed to investigate the behavior of interface stresses. The numerical results exhibit relaxation of interface stresses and large stress gradients, which are observed in the vicinity of the free surface. Since the exceedingly large stresses cannot be borne by the polymeric coating layer, local cracking or delamination
can occur at the interface corner.
KEYWORDs : thermal stress, coating layer, polymeric layer, interface stress
AUTHORs : Sang Soon Lee†

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