Determination of the critical potentials for pitting, protection, and stress corrosion cracking of 67-33 brass in fluoride solutions
- National Tsing Hua Univ., Hsinchu (Taiwan, Province of China). Dept. of Materials Science and Engineering
Pitting potentials and protection potentials have been determined for {alpha}-brass (33% Zn) using a cyclic polarization method. Increasing the F{sup {minus}} concentration shifts the critical potential to more active values. The pitting potentials, E{sub p}, and protection potentials, E{sub pp}, depend on the logarithmic concentration of F{sup {minus}} ions according to the equations: E{sub p} = a + b log [F{sup {minus}}] and E{sub pp} = a + b log [F{sup {minus}}]. Slow strain rate tests (SSRT), at a strain rate of 5 {times} 10{sup {minus}5} s{sup {minus}1}, were performed under open-circuit and potentiostatic conditions to study the stress corrosion cracking (SCC) characteristics of the {alpha}-brass in NaF solutions of various concentrations (pH 6.8) at 25 C. The minimum concentration of NaF that caused intergranular stress corrosion cracking (IGSCC) was 1 {times} 10{sup {minus}4} M. This concentration, was also the critical level for repassivation, observed in cyclic polarization tests. These results demonstrate a good correlation between the electrical and the mechanical breakdown of the passive film. In the presence of 10{sup {minus}1}M NaF the potential range for IGSCC was {minus}150 to {minus}50 mV (SCE). These critical potentials were restricted to the stable passive potential range and also fell within the potential-pH region where Cu{sub 2}O was stable. The formation of a Cu{sub 2}O film on the brass after polarization in the passive region was confirmed by X-ray diffraction (XRD). At more noble potentials and at cathodic potentials below the Cu{sub 2}O domain, the failure mode was ductile fracture. These observations of IGSCC of the brass in fluoride solutions support a film rupture-dissolution mechanism.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 40307
- Journal Information:
- Journal of the Electrochemical Society, Journal Name: Journal of the Electrochemical Society Journal Issue: 3 Vol. 142; ISSN 0013-4651; ISSN JESOAN
- Country of Publication:
- United States
- Language:
- English
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