Corrosion rates of mild steel in chromate solutions containing sulfate or chloride ions were estimated at a pH of 8 by the polarization resistance method. The critical concentration of chromate above which corrosion rate is below 1mdd increased with increase of sulfate or chloride concentration. Presence of Ca2+ ions did not affect the value of the critical concentration. It was demonstrated for waters from different sources that a rough estimation of the critical concentration of chromate could be made preliminarily as far as the concentrations of sulfate and chloride are known.
Characteristics of loose polyethylene sleeving in protecting buried cast iron pipelines from corrosion with and without the combined use of cathodic protection have been studied both in laboratory and field. Although the sleeving does not completely prevent water from entering onto the iron surface, it is effective in reducing corrosion and particularly pitting, if local defects exist in the film. Accelerated corrosion does not occur at and around a damaged film area. Cathodic current further ensures the protective effect by cathodically protecting exposed areas and by raising pH of water under the film.
A trial construction of battery operated mini potentiostat for potentiostatic test in field was performed by using an OP amplifier. A circuit diagram was presented and the detail of construction was described. Potentiostatic behaviors of mild steel were examined in fresh and polluted sea water, and it was certified that this potentiostat operated satisfactorily in both laboratory and field test. A modification for raising the input impedance of the instrument was briefly described.
A laboratory method for maintaining the dissolved oxygen (DO) content in corrosion media at any fixed levels below 100ppb was investigated. The nitrogen gas containing a known amount of oxygen gas generated by electrolysis of water with a galvanostat was continuously passed through deionized water and 6% NaCl solution at a temperature of 20°C, and the DO contents were measured by applying both the Winkler analysis and the polarographic method. A equation was presented to calculate the DO content using the value of applied electrolytic current. The results showed that the DO contents analyzed by the Winkler method agreed with those estimated by the equation with the precision of 95% or above for pure water and 90% or above for 6% NaCl solution, and it was certified that this method was very useful in laboratory application for controlling the DO content. The polarographic DO meter made in our laboratory could accurately measure the DO content. Some simplification of calculating equation was discussed. The end point of titration for the Winkler analysis could be precisely determined by applying amperometric titration with Pt and W electrode.