Corrosion of Mild Steel and Galvanized Steel in SO₂ Enclosures: The Incidence of Negative Gravimetric Corrosion Rates

Corrosion rates are frequently calculated from the weight loss of material samples, and they provide a measure of the degree of material degradation that has occurred when exposed to corrosive environments. However, some metal samples that have been exposed to corrosive environments experience negative weight loss, or more accurately, positive weight gain, which results in a negative corrosion rate. In the corpus of research on corrosion studies, there is little evidence for the occurrence of negative gravimetric corrosion rate in metals.

In this work, we employed gravimetric analysis to study the atmospheric corrosion of mild steel and galvanized steel in Sulphur (IV) oxide (SO₂) enclosures or chambers for a period of 2 weeks and 4 weeks. The results indicated weight gain of the metals after exposure to the SO₂-polluted atmospheric environment in the enclosures, thereby leading to negative corrosion rates. In seeking more insight to explain the observed phenomenon, XRF (X-Ray Fluorescence), SEM (Scanning electron microscopy) and FTIR (Fourier Transform Infrared Spectroscopy) analyses were conducted on the metal coupons.

While the XRF results show a consistent reduction in the iron (Fe) content of the samples with a lesser percent iron composition observed with increasing exposure time, the SEM results reveal the formation of crystalline corrosion products on the metal surfaces. The FTIR results also indicated the pronounced presence of hydroxyl functional groups.

Both the XRF and SEM results indicate that the active components of the metal samples are being used up in the surface electrochemical reactions and are converted to visible corrosion products which are responsible for the weight gain. Concluding from the FTIR results, the presence of corrosion products Fe(OH)₂ and Fe(OH)₃ is confirmed among others.