The influence of cyclic strain on the corrosion rate of cast steel was studied for 90 days in tap water with potassium chromate, (I), and with sodium nitrite, (II). Unstrained specimens were nearly protected from corrosion in solutions (I) and (II). However, heavy corrosion was found at the stress concentrated area of (I) in tests where a cyclic strain was imposed. Negative current loops in anodic polarization curves were observed in (I) for specimens finished with emery cloth. No negative current loops were observed for the rusted specimens. No negative current loops in anodic polarization curves were observed either in finished or rusted surfaces in (II). The following two conclusions can be derived from the test results. The cast steel was initially in an unstable passive state in the potassium chromate solution. The presence of the low cyclic strain disrupted the formation of a passive film, and corrosion heavily progressed. Anodic polarization curves in the presence of anodic inhibitors are useful to predict whether corrosion will progress or not under a cyclic strain condition.
As observed, Type 316 stainless steel with a low carbon content (Type 316L SS) sensitized at a temperature between 550°C and 700°C and submitted to the 65% nitric acid test, suffers from very severe intergranular corrosion (IGC). However the IGC mechanism of this alloy has not been well explained by the well known chromium depleted zone theory. In this study, the IGC mechanism of Type 316L SS in the 65% nitric acid test is discussed based on the analysis of grain boundary precipitates and the observation of the dissolution behavior of these precipitates. The obtained results are as follows; 1) IGC of sensitized Type 316L SS is not caused by the grain boundary precipitation of the σ phase (FeCr) but by the Laves [(Fe, Cr)2Mo] and the χ phases (Fe18Cr6Mo5). 2) IGC is caused by the dissolution of Laves and χ phases.
In order to generalize GI (General Corrosion Resistance Index) derived from the effects of alloy contents on the corrosion rate of stainless steel, the GI for Fe-Cr-Ni-Mo-Cu system stainless steels in various concentrations of sulfuric acid was introduced. It was clarified that the type of functional formula between general corrosion rate and GI did not change with the concentration of sulfuric acid. The coefficient of alloying elements in GI equation, however, varied depending upon the sulfuric acid concentration, and also the hydrosulfate ion and hydrogen ion dissociated from the sulfuric acid affected both reaction of metal-dissolution and of hydrogen-ion-reduction.
The exposed specimens in a marine atmosphere for 10 years were examined to know the pitting behavior of stainless steels. The linear relation was observed between the logarithm of the maximum pit depth and the logarithm of the exposure year, and the slope of the line was around 0.6. The pitting resistance of stainless steels depends on the alloy index, [Cr]+2[Mo], which was derived from multi-regression analysis. Here [Cr] and [Mo] mean chromium and molybdenum concentration in mass% respectively. The same pitting behavior was observed in the specimens tested with the newly developed cyclic corrosion test. The cyclic conditions are as follows: a synthetic seawater splay (14.4ks), drying (7.2ks), and wetting (7.2ks). The maximum pit depth in the actual environment can be estimated from the results of this test.
Initial oxide films formed on electrodeposited zinc (EG) and zinc-iron alloy (Zn-7 mass% Fe) coatings and zinc-iron alloy coating produced by thermal alloying of zinc electroplated steel (EGA) were analyzed using AES, XPS and TEM in order to elucidate electrochemical behavior of the zinc and zinc-iron alloy coatings. The outmost layer of oxide film for EG and EGA was composed of Zn(OH)2 and adsorbed H2O, and that for Zn-7 mass% Fe of Zn(OH)2, a small amount of Fe and adsorbed H2O. The inner layer of oxide film was composed of the following; predominantly ZnO for EG, predominantly amorphous Zn-(OH)2 and a small amount of Fe for Zn-7 mass% Fe, and predominantly ZnO and a small amount of Fe for EGA. The chemical state of Fe in the film could not be determined, but Fe is presumably oxidized. The suppression of oxygen reduction reaction on Zn-7 mass% Fe coating was considered to result from barrier effect against electron transfer in insulating Zn(OH)2 uniformly formed on the coating. The suppression action does not occur on EG and EGA coatings because of the formation of n-type semiconductive ZnO on the coatings.
Principle and applications of Photo Acoustic Spectroscopy (PAS) are briefly introduced. Applications of Photo Acoustic (PA) method for corrosion research, especially for nondestructive evaluation of under film corrosion of coated steels are described. Some results and their explanation demonstrate the capability of this method.
This paper reviews the corrosion of metallic restorations in the oral environment and its adverse effects on patients. Typical corrosion damages of metallic restorations observed in the oral cavity were shown and the factors affecting the biodegradation of these restorations were discussed. The biological side effects reported in the literature, especially an allergy to metallic ions dissolved from the restoration were described.