2019-01

Journal of The Electrochemical Society (Issue : 11) (Volume : 166)

Atmospheric corrosion of stainless steel can take place when airborne salt particles deposit on the metal surface, forming droplets when the relative humidity (RH) reaches a critical value: the deliquescence relative humidity of the salt. Most work to date has focused on single salts such as MgCl2 or NaCl. In the present work, the effect of mixed salts is investigated at 45% RH, above the deliquescence relative humidity of MgCl2 but below that of NaCl. Dish-shaped pits were found in pure MgCl2 solutions and mixed solutions. Crevice corrosion takes place under NaCl crystals. This is shown both with ex situ measurements and in situ time-dependent measurements using X-ray microtomography, where pit growth was also monitored.

2019-01

Corrosion Science (Volume : 150)

Relative humidity fluctuations tend to influence atmospheric corrosion under natural exposure conditions. In this study, the effect of change in relative humidity on pitting corrosion of stainless steel under MgCl2 droplets is investigated with in situ X-ray microtomography and optical microscopy. Relative humidity fluctuations (between 33% RH and 85% RH or between 33% RH and 12% RH) tend to lead to nucleation of many small pits whereas continuous exposure at constant 33% RH leads to growth of a single pit. This indicates that natural fluctuations in relative humidity might be beneficial for preventing the growth of a large penetrating pit.

2018-09

CORROSION (Issue : 12) (Volume : 74)

Atmospheric corrosion of stainless steel is of concern for intermediate level nuclear waste (ILW) containers. The effect of microstructure on the morphology of atmospheric corrosion pits in Type 304L stainless steel plate was investigated on three orthogonal planes under MgCl2 droplets. Pits on the top surface of the plate show ring-like structures, whereas pits on the plate sides show a striped morphology. Synchrotron x-ray tomography of Type 304L stainless steel pins shows the presence of similar striped attack. Scanning electron microscopy on plate samples revealed the presence of parallel bands along the rolling direction. Energy dispersive spectroscopy maps and line scans across these bands indicated a local increase in the Cr/Ni ratio consistent with a ferrite phase, likely residual delta-ferrite formed during solidification. Vibrating sample magnetometer (VSM) detected the presence of ferrite on the base alloy. X-ray diffraction and electron backscatter diffraction quantified the volume fractions of ferrite and austenite phases. Ferrite phases affect the morphology of pits and promote pit propagation along the rolling direction.

2018-05

CORROSION (Issue : 5) (Volume : 74)

Pit location during atmospheric corrosion of Type 304L stainless steel under MgCl2 droplets depends on initial droplet concentration. Pits formed predominantly in the center of the droplet for concentrations ≥4 M, closer to the perimeter for 1.5 M to 3 M, and were randomly distributed for concentrations ≤1 M. Pits initiated only after the droplets had evaporated to a critical concentration >3 M, where droplets deposited with lower initial concentrations were thinner. The results can be explained in terms of “differential aeration” and IR drop effects, showing that corrosion in “splash zones” may differ from that under aerosol salt deposit layers that deliquesce forming initially saturated solutions.

2015-01

Faraday Discussions (Volume : 180)

The morphology of atmospheric pitting corrosion in 304L stainless steel plate was analysed using MgCl2 droplets in relation to changes in relative humidity (RH) and chloride deposition density (CDD). It was found that highly reproducible morphologies occur that are distinct at different RH. Pitting at higher concentrations, i.e. lower RH, resulted in satellite pits forming around the perimeter of wide shallow dish regions. At higher RH, these satellite pits did not form and instead spiral attack into the shallow region was observed. Increasing CDD at saturation resulted in a very broad-mouthed pitting attack within the shallow dish region. Large data sets were used to find trends in pit size and morphology in what is essentially a heterogeneous alloy. Electrochemical experiments on 304 stainless steel wires in highly saturated solutions showed that the passive current density increased significantly above 3 M MgCl2 and the breakdown pitting potential dropped as the concentration increased. It is proposed that the shallow dish regions grow via enhanced dissolution of the passive film, whereas satellite pits and a spiral attack take place with active dissolution of bare metal surfaces.

2014-09

Corrosion Engineering, Science and Technology (Issue : 6) (Volume : 49)

Atmospheric pitting corrosion of stainless steel was examined with synchrotron X-ray microtomography, laboratory based tests and electrochemical measurements in order to provide a mechanistic basis for the development of corrosion prediction models. It was found that the morphology of corrosion pits was affected by the presence of residual ferrite, and some pits showed dense covers that may affect pit stability. Fluctuations in relative humidity may lead to partial or complete repassivation of pits, sometimes leading to the formation of new pits. Electrochemical measurements in artificial pits containing concentrated salt solutions show a strong dependence of the diffusion limited current density on chloride concentration.