Environmentally-Assisted Cracking of Engineering Materials - An Insight

Abstract

This paper reviews the environmentally-assisted cracking studies of some of the key engineering materials i.e. austenitic stainless steels, aluminium alloys and magnesium alloys. Some of the salient work carried out by the research laboratories and institutions in India and other countries, in this critical research area, are brought out in this paper. Environmentally-AssistedCracking of Engineering Materials known as EAC. Stresses arise in practice from applied load or from residual stress due to welding or inhomogeneous plastic deformation.The literature on EAC of engineering materials is vast, and there are many useful textbooks and conference proceedings [1][2][3][4][5]. Understanding the EAC mechanism(s) in metals has been a subject of interest for decades.Basically, the EAC can be broadly classified into two groups: (i) anodic dissolution assisted cracking and (ii) hydrogen-induced cracking. Many models have been put forward, in literature, to explain the EAC mechanism(s). The slip-dissolution model is one of them explaining the crack growth occurring by extremely localized dissolution of the metal. This model states that the crack is protected by a film, usually an oxide, which is fractured as a result of plastic strain in the metal at the crack tip. The crack growth proceeds by a cyclic process of film-rupture, dissolution and filmrepair. This model has been mainly cited to explain the intergranular cracking of femtic steels in passivating environments such as carbonate-bicarbonate solutions as well as sensitised stainless steels in a variety of environments.Later, a film-induced cleavage model was proposed to explain the brittle failure of alpha-brass due to dezincification in ammonia solution. Hydrogeninduced cracking or hydrogen embrittlement in metallic materials is also widely accepted as a mechanism of failure in recent years. Hydrogen discharge and absorption are favoured when there is acidification of the local environment by cation hydrolysis, especially if this acidification damages or destroys a passive film in the crack. Once hydrogen is absorbed in the metal, it can promote cleavage, intergranular separation, or a highly localized plastic fracture. In hydride-forming metals such as Mg and Ti, formation of brittle hydrides can be part of the fracture mechanism.The research organisations and institutions in India have contributed significantly to the understanding of EAC behaviour of engineering materials. Some of the key research works carried out in this research area by