Atmospheric Corrosion and Stress Corrosion Cracking of Stainless Steels Understanding the Evolution of Corrosion/Cracking and Threshold Parameters

摘要

Atmospheric corrosion remains a particular concern for materials subjected to environments inwhich salt-laden particulates can deposit onto the surface and form, in the presence of humidity,aggressive thin-film electrolytes. In the presence of stress, the transition to atmosphericallyinducedstress corrosion cracking (AISCC) can occur and endanger the life time of structuralmaterials. In recent works, the corrosion morphology beneath droplets has been shown to be rathercomplicated; pits, crevices, selective, intergranular and filiform corrosion can all occur beneathdroplets at the same time/deposited area, with cracks further complicating the matter. Stresscorrosion cracking (SCC) as well as hydrogen embrittlement (HE) have been observed underchloride-laden droplets. The latter has been less discussed as such cracks were typicallyassociated with SCC, although HE can also operate in highly-acidic conditions, which are typicallygenerated within droplets. This paper will discuss crack and corrosion morphologies observed inaustenitic/duplex stainless steels upon exposure to MgCl_2 and FeCl_3/MgCl_2 droplets, and will showthat both cracking mechanisms can operate, with the hydrogen believed supporting the SCCprocess. The paper will further deal with monitoring and characterisation of the corrosion processand will express the need for in-situ, real-time measurements. X-ray computer-aided tomography(XCT) has become routine practise in laboratories which can be very helpful in assessing the timefor corrosion/crack initiation and visualise and quantify their propagation.Threshold parameters have been often used to characterise atmospheric corrosion behaviour andto describe a safe usage for long times, such as the critical pitting temperature (CPT), a wellestablishedparameter describing the temperature below which pits cannot stably grow. However,other than from pits, AISCC cracks may also be initiated from crevices which requires rather thecritical crevice temperature (CCT) than the CPT to be considered. Furthermore, the thresholdchloride deposition density (CDD) was introduced to express the quantity of chloride necessary forcorrosion. This, however, remained not so straightforward as the CDD threshold typically assumesa continuous and homogeneous droplet, which certainly is not true as secondary spreading, i.e. thedroplet growth beyond its initially deposited area during exposure, can change the CDD and falsifya quantitative approach. There is also the threshold stress which, however, is insufficient as thecorrosion aspect is not included. It is perhaps the threshold time which has rather significance inlong-term exposures such as for container materials of nuclear waste, which are required to besafe over tens of decades. The threshold concept will be discussed in light of finding a betterdescription for quantitative assessment of atmospheric corrosion.