Crevice Corrosion - NaCI Concentration Map for Grade-2 Titanium at Elevated Temperature
- PDF / 340,896 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 25 Downloads / 150 Views
CREVICE CORROSION - NaCI CONCENTRATION MAP FOR GRADE-2 TITANIUM AT ELEVATED TEMPERATURE
SHIGEO TSUJIKAWA AND YOICHJ KOJIMA The University of Tokyo, Department of Metallurgy, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113 JAPAN
ABSTRACT
The repassivation potential, ER, for metal/metal-crevice of Commercially Pure Titanium, C.P.Ti, was determined in NaC1 solutions at temperatures up to 250C. The ER has its least noble value near 100C and becomes more noble as the temperature increases. As shown in previous research[l], the shrinkage of the repassivation region should continue with increasing temperatures. However, in conducting this same experiment at temperatures higher than 100C, an examination of the NaCI concentration - temperature - crevice corrosion map verifies that the repassivation region began to expand again when the temperature exceeded 140C. This expansion continued as the temperature continued to increase.
INTRODUCTION
The repassivation concept[ 1] was reapplied to evaluate the long term corrosion resistance of titanium at elevated temperatures. The crevice corrosion map, Figure 1, of the previous paper[l] shows the determined boundary lines for the temperature below 100C. These lines were further extrapolated to higher temperature ranges. The boundary lines were based upon experimentally determined points whose attributes are the repassivation NaCl concentration, CNaCI,R, and the repassivation temperature, TR, in the ambient pressure. When using a package in an environment where the temperature was above 100C, it was necessary to determine the actual critical condition by conducting the experiment for the desired temperature. Therefore, in this paper, the repassivation potential, ER, for metal/metalcrevice of Commercially Pure Titanium, C.P.Ti, was measured in the NaCl solutions at elevated temperatures ranging up to 250C, and used to deduce the critical condition. 300 0.01%Fe UI
1
0 .0001
%Fe '
200 -
&100.
--
0.09-
G-12 Crevice \corrosion
\
Figure 1 Crevice Corrosion Map in Terms of NaCl Concentration and Temperature for C.P.Ti with Fe Content of 0.01% / More than
S\
~
Repassivation
0.09% in Comparison with G-12Ti. .001
.01
.1
1
10
100
NaCl (%) Mat. Res. Soc. Symp. Proc. Vol. 294. (c1993 Materials Research Society
312
EXPERIMENTAL METHOD
The C.P.Ti used in this paper was identical to the Sample #15 alloy (Fe; 0.07, 0; 0.18, N; 0.008, H; 0.002, C; 0.006 (wt.%)) in the previous paper [1]. The crevice assembly is formed by a bolt and two nuts. The contact surface of the two Ti alloy nuts, inner diameter 10mm and outer diameter 20mm, provide the metal/metal-crevice. The would-be crevice surfaces of the two nuts were wet-finished with #1500 SiC paper and finger-tightened on the Ti bolt. The electrode potential is expressed with respect to the saturated calomel electrode, SCE, at room temperature. After the crevice specimens were held at -200mV in a NaCl solution of 1 - 25%, at 100C for 24 hours, it was observed that the crevice corrosion grew deeper than the critical depth, h* [1]. An uppe
Data Loading...
