The structure of the electric power business has been undergoing a major transformation in recent years. Japan's vertically integrated electric power supply system is now going through a historical paradigm shift with respect to the wide-area operation of power transmission lines, complete liberalization of electricity retailing, and separation of power generation and transmission. These trends also affect the issue of preventing the corrosion of power distribution facilities, making efforts to maintain the facilities and to estimate their service life and damage risk all the more important. This paper focuses on the issue of preventing the corrosion of utility poles and outside steel structure facilities that account for a large portion of the installed electric power distribution network. It describes the present status of performance evaluation tests conducted on prestressing steel tendons used in concrete poles (PC steel) and heavy duty coatings as well as the activities of related academic societies.
A 3-week exposure test of the C-ring specimen sampled from a commercial phosphorous deoxidized copper tube in ammoniacal environment were carried out to investigate stress corrosion cracking of a pure copper tube. C-ring specimens were applied high and low stresses; a high stress was over a proof stress, whereas a low stress was within an elastic region. Stress corrosion cracking occurred in the highly stressed C-ring specimen after the 3-week exposure test in gas-phase ammonia derived from 3 and 9wt% ammonia water. On the other hand, intergranular corrosion was observed in the C-ring specimens under low stress. A diameter recovery rate of C-ring specimen for which stress corrosion cracking developed was smaller than that of C-ring specimen without stress corrosion cracking.
The electrochemical tests instead of the immersion tests for evaluating a crevice corrosion resistance of stainless steel are used. One of the electrochemical tests is the test method of JIS G 0592 that is the cyclic polarization test with the stepwise potential backscan. However, the propagation behavior of crevice corrosion during this test is unclear enough; e.g., the corrosion behavior in crevice at each step (step I-step IV) of the test procedure or under the potential of the critical potential, E'R,CREV, determined by JIS G 0592. When the corrosion depth of the specimen after the test is more than 40μm, E'R,CREV is renamed ER,CREV, the repassivation potential.
In this paper, in-situ observation of the crevice corrosion propagation behavior in the quartz-glass/metal crevice of SUS 304 and SUS316L was carried out at each step of the test procedure from step I to step IV. Main results were as follows;
(1) A crevice corrosion occurred at the step I, potentiodynamic stage, and the early stage of step II, galvanostatic stage. The crevice corrosion spread in the crevice and penetrated into the depth at the edge of crevice mouth until the end of step II.
(2) The crevice corrosion propagated as increasing in corroded area at step III, potentiodynamic backscan stage, and at step IV, stepwise potential backscan stage. Furthermore, the crevice corrosion area continuously increased even under the potential of E'R,CREV determined at stage IV. The propagation of crevice corrosion stopped under the potential at which there was no expansion in corroded area.
Recently, it has been a problem that ant's nest corrosion occur in phosphorus deoxidized copper (hereinafter called PDC) tubes for indoor equipment in room air-conditioner. We've developed zinc plating copper tube on sacrificial anticorrosion effect to ant's nest corrosion. The corrosion resistance of the tube was evaluated compared with corrosion properties of PDC and oxygen free copper (hereinafter called OFC) by vapor phase exposure test in a sealed container having a corrosive medium of 100 mg/L formic acid solution. In addition, it was also evaluated corrosive properties of OFC. Zinc plating copper tube has been completely suppressed ant's nest corrosion during corrosion test. We considered that OFC has slow corrosion property in the depth direction, because corrosion source part at bottom of corrosion hole in OFC is less than in PDC.
Phosphorous deoxidized copper tubes were immersed in deaerated and aerated 0.002 mol/L formic acid solutions for 5 days to investigate the influence of dissolved oxygen on corrosion behavior of copper in formic acid solution. The dark brown film, suspected to be a cuprous oxide, formed on the surface of the specimen immersed in aerated test water, whereas the metallic luster of the surface of the specimen immersed in deaerated test water was retained up to 5 days. The pH value of the formic acid solution in which copper tubes were immersed increases with increasing immersion period. A rise of pH value is inhibited by deaeration of test solution. The concentration of dissolved copper in deaerated solution continues to rise gradually over immersion time. On the other hand, the concentration of dissolved copper in aerated solution remains at a certain value after the steep rise. This is probably due to the redeposition of dissolved copper on the surface of the specimen as cuprous oxide film. Ant nest corrosion occurred in the copper tubes immersed in aerated formic acid solution for 28, 56 and 84 days. It is concluded that dissolved oxygen is one of the indispensable factors for ant nest corrosion of copper.
To clarify the corrosion inhibition performance of oxygen reducing treatment for copper tubes of air-conditioning heat source machines at an open heat storage water system, reports on eddy-current inspection for copper tubes of the heat source machines and dissolved oxygen (DO) concentrations of chilled water and hot water during the operation were re-analyzed. When a copper tube was damaged more than 5% of its original thickness, it was defined as the damaged copper tube and the ratio of the number of damaged copper tubes against the number of eddy current inspected copper tubes was calculated as the damage rate. It was found that the damage rate of copper tubes of the heat source machine was decreased by reducing DO concentrations, and it could be obtained less than 1% when DO concentration is controlled less than 1mg/L.
SUS304 and SUS430LX stainless steels were subjected to cyclic voltammetry (CV) in EMI-TFSI ionic liquid containing water of 5.5, 30 and 5000 ppm at room temperature. Structural changes of passive film during CV was characterized by AES and TEM/EELS. Passive films after CV were mainly composed of mainly Cr and Fe oxides similar to that formed in aqueous solutions. The thickness and Cr cation fraction of passive film increased with increasing CV cycles. Furthermore, much Cr concentrated and thinner passive films were formed with less water content in EMI-TFSI. It is concluded that SUS304 and SUS430LX stainless steel are corrosion resistant in EMI-TFSI ionic liquid with small amount of water.