Abstract
Pump shafts are used being fitted with impellers and sleeves. Since they are often made from different metals and form crevices, galvanic corrosion or crevice corrosion occurs in a corrosive solution. Therefore, corrosion fatigue strength (CF) of these components may differ from that of bare metal specimen. In this paper, effects of galvanic couples and crevices on CF were investigated. Round bar materials tested are a 0.35%C steel (S35C) and a 13Cr stainless steel (SUS 403). Sleeves used are Cu, Al and S35C for S35C specimen and brass and SUS 403 for SUS 403 specimen. All tests were carried out at 1450 or 2770rpm in 3% NaCl solution. Test results are summarized as follows.
(1) The corrosion fatigue strength σwc of a S35C at N=107 was 10kgf/mm2 (33% of that in air) and considerably decreased at higher cycles. For SUS 403 σwc was 19.6kgf/mm2 (59% of that in air) and decreased little at higher cycles.
(2) The effect of sleeve materials on CF of S35C bar specimen was explained by considering the difference in corrosion potentials between the specimen and the sleeve. Namely, the highest σwc was obtained for Al sleeve (12.5kgf/mm2 at N=2×107), followed by S35C sleeve (10kgf/mm2: the same as that in bare specimen) and Cu sleeve (6.7kgf/mm2).
(3) In the case of SUS 403 bar specimen, the same σwc was obtained for two different sleeve materials, while both SUS 403 and brass sleeve considerably decreased σwc because of the formation of corrosion pits at the ends of the sleeves. (10kgf/mm2 at N=5×107, 30% of that in air)
(4) To clarify the effect of pit on CF, corrosion fatigue test of etch pitted round bar specimens was carried out, in which etch pits were induced by the potential method and the size of the pits was controlled to be as large as those found on fracture surface of the sleeve fitted specimens. It was shown that σwc of etch pitted specimens was roughly equal to that of the sleeve fitted specimen in the high cycle regime.
