Corrosion Behaviour of a Cr2O3 Coating on Mild Steel in Synthetic Mine Water

Abstract

The Cr2O3 coating on the surface of ASTM A516 Grade 70 mild steel substrates was developed using the thermal plasma spraying process for protection against corrosion and wear. The microstructural behaviours for both coating and substrate were analysed using SEM and XRD techniques. The corrosion behaviours of the coatings and substrate in synthetic mine water with varying pH values (6, 3, and 1) were evaluated according to ASTM standards for potentiodynamic polarisation measurements. Tafel plots were drawn to determine the corrosion rates. Vickers hardness of the coatings and substrate were measured. The Cr2O3 coating exhibited cracks due to the solidification and cooling process, as well as some pores between the top and bonding layers caused by unmelted or partially melted particles. The corrosion tests revealed that a decrease in pH levels led to increased corrosion rates in both samples. The Cr2O3 coating demonstrated superior corrosion resistance, ranging from <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" id="M1"><a:mn>0.036</a:mn><a:mo>±</a:mo><a:mn>0.003</a:mn></a:math> mm/year to <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" id="M2"><c:mn>0.110</c:mn><c:mo>±</c:mo><c:mn>0.004</c:mn></c:math> mm/year, compared to the mild steel substrate, which ranged from <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" id="M3"><e:mn>0.262</e:mn><e:mo>±</e:mo><e:mn>0.021</e:mn></e:math> mm/year to <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" id="M4"><g:mn>0.336</g:mn><g:mo>±</g:mo><g:mn>0.026</g:mn></g:math> mm/year, across all pH values. Moreover, it exhibited significantly greater hardness ( <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" id="M5"><i:mn>1260</i:mn><i:mo>±</i:mo><i:mn>77</i:mn></i:math> HV3) than the mild steel substrate ( <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" id="M6"><k:mn>180</k:mn><k:mo>±</k:mo><k:mn>14</k:mn></k:math> HV3). The lower corrosion rates and higher hardness of Cr2O3 coating than the mild steel substrate make it a suitable coating in applications where corrosion resistance and high hardness properties are essential.