Studies of passive films on stainless steels are reviewed from the viewpoint of developing new resources-saved stainless steels (green stainless steels). Starting from the overview of the results of in-situ analyses for the thickness and chemical composition of real passive films, there are describing some important findings obtained from the model experiments using artificial passive films and ion-beam-sputter deposited Fe-Cr thin films, which simulate real passive films and ideal stainless steels, respectively. Finally, recent studies on the pit initiation mechanism at sulfide inclusions and on the improvement of pitting resistance by removal of the inclusions from steel surface are introduced to demonstrate a possibility of developing green stainless steels.
The effects of small amount of tantalum addition to a Ni-Cr-Mo alloy on crevice corrosion resistance in 25% NaCl at 107°C was investigated quantitatively by measuring the repassivation potentials for corrosion crevice (ER,CREV). Upon the results, tantalum was revealed to shift the repassivation potentials to the noble direction and the effect was much more than the chromium or molybdenum. For the tantalum free alloys, the crevice corrosion resistance was found to correlate closely with the Pitting Resistance Evaluivalent number (PRE=[%Cr]+3.3([%Mo]+0.5[%W])), however, the tantalum containing alloys were against the correlation. Consequently, PRE(Ta)(=[%Cr]+3.3([%Mo]+0.5[%W])+7.7[%Ta]) was induced by adding a term of [%Ta] to the PRE equation. With this PRE(Ta), the crevice corrosion resistance for the tantalum containing alloys can be estimated.
Corrosion is one of the most important problems, particularly in relation to outdoor telecommunication equipment. Some corrosion prevention techniques such as crevice corrosion prevention for stainless steel and protective coatings on galvanized steel have been developed. As regards the latter, we have already developed a duplex protection system that employs resin coating and a galvanizing technique. There has been a need for weathering resistance technology that can protect telecommunications plant materials against damage during long-term use. In this work we have tried to improve the performance under various weathering conditions, in particular under ultraviolet (UV) light, which can easily damage polymer materials. Some powder paints designed to protect against UV light were prepared, and their performances were evaluated both with accelerated UV testing (e.g. with a xenon arc device) and when subjected to natural sunlight outdoor exposure in Okinawa, which has the most severe UV light conditions in Japan. We found that the UV absorber clearly improved the resistance to UV weathering. In addition, we tested for corrosion resistance under severely corrosive conditions outdoors at a coastal location on Miyakejima Island, and found that no blistering, cracking or corrosion occurred at current exposure time.