EFFECT OF MECHANICAL AND CHEMICAL FACTORS ON THE CORROSION OF SOME COPPER- BASE ALLOYS

Abstract

The main objective of this thesis is to study and compare the corrosion behavior of modified Al-brass (MA72) and 90/10 Cu-Ni alloys and to evaluate their susceptibility to stress corrosion cracking (SCC) and corrosion fatigue (CF) in sodium chloride solution containing different concentrations of sodium sulfide (100-- 1000 ppm). For this purpose the following measurements were caried out : - Potential-time , polarization , weight variation measurements, X-ray diffraction analysis and microscopic examinationfor the two alloys in 3.5% NaCl in absence and in presence of different concentrations of sulfide ( I 00- 1000 ppm) - sCC tests using the constant slow strain rate technique for both alloys in the same solutions. - Corrosion fatigue (CF) tests using cyclic bending technique for the two alloys in the above mentioned solutions. - Microscopic investigations (SEM) for the corroded or failed specimens of the two alloys. The results indicated that the corrosion behavior of Al-brass and Cu-Ni in pure 3.5% NaCl has small difference , but Cu-Ni alloy demonstrated better corrosion resistance . In presence of sulfide ions , the corrosion of both alloys has increased considerably. Moreover , pitting corrosion in case of Cu-Ni and intergranular corrosion in case of Al-brass was also detected in presence of high concentrations of sulfide (500-1000 ppm). Good corrosion resistance of the two alloys in 3.5 % NaCl solution was attributed to the formation of a protective oxide film (predominantly Cu,0) which is much better in case of Cu-Ni alloy due to doping with Ni .In presence of sulfide ion the corrosion resistance was considerably decreased due to the formation of CuS which interferes with the protecting oxide film and reduces its corrosion resistance . The detrimental effect of sulfide on the corrosion resistance increases