Stress corrosion cracking and hydrogen embrittlement of an Al-Zn-Mg-Cu alloy

The age hardening, stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of an Al-Zn-Mg-Cu 7175 alloy were investigated experimentally. There were two peak-aged states during ageing. For ageing at 413 K, the strength of the second peak-aged state was slightly higher than that of the first one, whereas the SCC susceptibility was lower, indicating that it is possible to heat treat 7175 to high strength and simultaneously to have high SCC resistance. The SCC susceptibility increased with increasing Mg segregation at the grain boundaries. Hydrogen embrittlement (HE) increased with increased hydrogen charging and decreased with increasing ageing time for the same hydrogen charging conditions. Computer simulations were carried out of (a) the Mg grain boundary segregation using the embedded atom method and (b) the effect of Mg and H segregation on the grain boundary strength using a quasi-chemical approach. The simulations showed that (a) Mg grain boundary segregation in Al-Zn-Mg-Cu alloys is spontaneous, (b) Mg segregation decreases the grain boundary strength, and (c) H embrittles the grain boundary more seriously than does Mg. Therefore, the SCC mechanism of Al-Zn-Mg Cu alloys is attributed to the combination of HE and Mg segregation induced grain boundary embrittlement. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an alkalisation, passivation, poisoning effect or shortcut effect. (C) 2003 Elsevier Ltd. All rights reserved.

Atom probe field ion microscope measurements of carbon segregation at an a:a grain boundary and service failures by intergranular stress corrosion cracking

This work reports on a critical measurement to understand the intergranular stress corrosion cracking (IGSCC) of pipeline steels: the atom probe field ion microscope (APFIM) measurement of the carbon concentration at a grain boundary (GB). The APFIM measurement was related to the microstructure and to IGSCC observations. The APFIM indicated that the GB carbon concentration of X70 was similar to 10 at% or less, which correlated with a high resistance to IGSCC for X70. (C) 2005 Elsevier Ltd. All rights reserved.

Stress corrosion cracking and hydrogen diffusion in magnesium

Evaluation of recent data for hydrogen (H) diffusion in magnesium (Mg) yielded a new equation for the diffusion coefficient of H in Mg. This indicates that there can be significant H transport ahead of a stress corrosion crack in Mg at ambient temperature and that H may be involved in the mechanism of stress corrosion cracking in Mg.

Atmospheric corrosion of copper and the colour, structure and composition of natural patinas on copper

This paper describes the results of atmospheric corrosion testing and of an examination of patina samples from Brisbane, Denmark, Sweden, France, USA and Austria. The aim was threefold: (1) to determine the structure of natural patinas and to relate their structure to their appearance in service and to the atmospheric corrosion of copper; (2) to understand why a brown rust coloured layer forms on the surface of some copper patinas; (3) to understand why some patinas are still black in colour despite being of significant age. During the atmospheric corrosion of copper, a two-layer patina forms on the copper surface. Cuprite is the initial corrosion product and cuprite is always the patina layer in contact with the copper. The growth laws describing patina formation indicate that the decreasing corrosion rate with increasing exposure time is due to the protective nature of the cuprite layer. The green patinas were typically characterised by an outer layer of brochantite, which forms as individual crystals on the surface of the cuprite layer, probably by a precipitation reaction from an aqueous surface layer on the cuprite layer. Natural patinas come in a variety of colours. The colour is controlled by the amount of the patina and its chemical composition. Thin patinas containing predominantly cuprite were black. If the patina was sufficiently thick, and the [Fe]/[Cu] ratio was low, then the patina was green, whereas if the [Fe]/[Cu] ratio was approximately 10 at%, then the patina is rust brown in colour. The iron was in solid solution in the brochantite, which might be designated as a (copper/iron) hydroxysulphate. In the brown patinas examined, the iron was distributed predominately in the outermost part of the patina. (c) 2005 Elsevier Ltd. All rights reserved.

Aspects of the prevention and repair of chloride-induced corrosion of steel in concrete

Sodium formate, potassium acetate and a mixture of calcium and magnesium acetate (CMA) have all been identified as effective de-icing agents. In this project an attempt has been made to elucidate potentially deleterious effects of these substances on the durability of reinforced concrete. Aspects involving the corrosion behaviour of embedded steel along with the chemical and physical degradation of the cementitious matrix were studied. Ionic diffusion characteristics of deicer/pore solution systems in hardened cement paste were also studied since rates of ingress of deleterious agents into cement paste are commonly diffusion-controlled. It was found that all the compounds tested were generally non-corrosive to embedded steel, however, in a small number of cases potassium acetate did cause corrosion. Potassium acetate was also found to cause cracking in concrete and cement paste samples. CMA appeared to degrade hydrated cement paste although this was apparently less of a problem when commercial grade CMA was used in place of the reagent grade chemical. This was thought to be due to the insoluble material present in the commercial formulation forming a physical barrier between the concrete and the de-icing solution. With the test regimes used sodium formate was not seen to have any deleterious effect on the integrity of reinforced concrete. As a means of restoring the corrosion protective character of chloride-contaminated concrete the process of electrochemical chloride removal has been previously developed. Potential side-effects of this method and the effect of external electrolyte composition on chloride removal efficiency were investigated. It was seen that the composition of the external electrolyte has a significant effect on the amount of chloride removed. It was also found that, due to alterations to the composition of the C3A hydration reaction products, it was possible to remove bound chloride as well as that in the pore solution. The use of an external electrolyte containing lithium ions was also tried as a means of preventing cathodically-induced alkali-silica reaction in concretes containing potentially reactive aggregates. The results obtained were inconclusive and further practical development of this approach is needed.

Corrosion and passivation of steel in concrete

A study of several chemical and electrochemical factors which affect the behaviour of embedded steel in cement pastes and concrete has been made. The effects of internal and external sources of chloride ions on the pore solution chemistry of Portland cement pastes, with and without additions of anodic corrosion inhibitors, have been studied using a pore solution expression device which has enabled samples of pore solution to be expressed from hardened cement pastes and analysed for various ionic species. Samples of pure alite and tricalcium aluminate have been prepared and characterised with respect to morphology, free lime content and fineness. Kinetics of diffusion of chloride ions in hardened pastes of alite and alite blended with tricalcium aluminate have been investigated and an activation energy obtained for the diffusion process in alite. The pore structures of the hardened pastes and the chloride ion binding capacity of alite have also been determined. Concrete cylinders containing embedded steel with four different surface conditions were exposed to various environments. The electrochemical behaviour of the steel was monitored during the period of exposure by means of rest potential measurements and the steel corrosion products analysed before and after being embedded. An examination was made of the nature of the interfacial zones produced between the embedded steel and cement. Rest potential measurements were monitored for steel embedded in alite paste in the presence of chloride ions and cement paste containing various levels of inhibitors in combination with chloride ions. In the latter case the results were supported by polarisation resistance determinations.

The effect of frequency and microstructure on corrosion fatigue crack propagation in high strength aluminium alloys

Fatigue crack growth in high strength aluminium alloy 7150 commercial plate material has been studied in both laboratory air and acidified aqueous salt solution. The aggressive aqueous environment enhanced fatigue crack growth rates by up to an order in magnitude compared to laboratory air. The enhancement in fatigue crack growth rate was accompanied by evidence of embrittlement in the crack path, involving both brittle intergranular and transgranular failure modes. Both the enhancement of fatigue crack growth rates and the extent of intergranular growth modes are dependent on cyclic frequency which, along with the absence of a similar frequency effect in a spray-formed version of the material with a significantly different grain structure, supports a mechanism of grain boundary hydrogen diffusion for intergranular corrosion fatigue crack growth. The convergence of corrosion fatigue crack growth rates at high ΔK in both spray-formed and conventional plate materials coincides with the operation of identical transgranular corrosion fatigue modes dependent on strain-controlled hydrogen diffusion ahead of the crack tip. © 1997 Acta Metallurgica Inc.