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Hydrogen embrittlement of an Al-4.5Zn-2.5Mg alloy

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Boran, Edward John (1989) Hydrogen embrittlement of an Al-4.5Zn-2.5Mg alloy. Doctoral thesis, City of London Polytechnic.

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Abstract

Experimental results are presented here for a 7017-T651 AlZnmg alloy where stress corrosion crack growth data on compact tension and double cantilever beam specimens has been obtained in distilled water, water saturated air vapour and aqueous chloride environments over a range of temperatures. The results, including fracture surface examination, are interpreted as indicating that a hydrogen embrittlement mechanism is operative whereby hydrogen, generated by a corrosion reaction, embrittles the alloy. Activation energy determinations have been made for stress corrosion crack growth at the free corrosion potential, and the activation energy results have been interpreted as indicating that the same process is rate controlling during both region I and region II crack growth.

Hydrogen diffusion in AlZnMg has been investigated by electrochemical permeation and gas chromatography techniques. It was found that the sensitivity of the electrochemical permeation technique is not sufficient to detect a permeation transient due to hydrogen. However, the diffusion coefficient for hydrogen in AlZnMg was determined by gas chromatography over a range of temperatures. A critique is made of previous reported measurements of hydrogen diffusion in an AlZnMg alloy by electrochemical permeation. The average diffusion rate of hydrogen in AlZnMg alloy material using gas chromatography on hollow cylindrical specimens was determined.

Finally, the surface reactions between polished AlZnMg alloy surfaces and distilled water, water saturated air vapour and aqueous chloride solutions have been Investigated to determine the rate of hydrogen production and the morphology of the reaction products. It was shown that the reaction of water saturated air vapour with the commercial AlZnMg alloy surface did not exhibit specificity with respect to grain boundary attach, unlike the specificity shown by high purity versions of these alloys. The contribution of these studies to the understanding stress corrosion crack growth by a hydrogen embrittlement mechanism is discussed.

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