Corrosion of steel in the marine splash-zone.

Shaw, David

Abstract

The effect of the marine splash-zone on corrosion rates of steels BS 4360 43A and 50D, Corten A and pure iron was assessed. Specimens were exposed at coastal and deep water sites as well as being subjected to splash-zone simulation in the laboratory. Gravimetric, microscopic and a variety of electrochemical techniques were used in the laboratory, to study the behaviour of exposed specimens. Corrosion rates were found to be higher for specimens exposed to initially wet conditions though high temperatures were found to correlate with high corrosion rates of thickly-scaled steels. Scales became compact and resistant to cathodic reduction as exposure times increased but corrosion rates tended to increase with exposure time. Polarization of scaled specimens in synthetic sea water revealed increased cathodic depolarization, anodic polarization and decreased dependence on oxygen concentrations to result, as exposure times increased. At open circuit potentials neither the anodic nor the cathodic reactions appeared to be under activation control. Coastal and simulator corrosion rates for the various metals' increase in the order: · Pure iron Corten A 50D 43A. The morphology of corrosion products was found to be alloy dependent. Simulator produced scales were found to be fragile but electrochemical tests showed them to produce similar effects to those noted on coastally exposed specimens. Monitoring of the open circuit potential during intermittent immersion revealed the manner in which cathodic depolarization increased with immersion time. High silicon levels due to surface contamination resulted in uniformly low corrosion rates and high pitting rates for the specimens exposed at the open sea site, though corrosion products resembled those coastally formed. It is proposed that the cyclic splash-zone corrosion process involves scale reduction during wet periods at a rate dependent on the surface area of the porous corrosion product along with oxygen reduction during drying periods when moisture films cover specimens. Corrosion scales formed were found to be thicker than typical atmospherically formed corrosion product layers. This difference is believed to be a function of the high concentration of electrolyte-containing pores in the scales. The simulator was found to be a useful tool as it allowed the effects of variations in the cyclic wetting process to be related to the electrochemical nature of corroding surfaces. This allowed the major flaws in the polarization resistance and Tafel extrapolation techniques to be identified. The simulator produced a condition less humid than the total immersion condition but more humid than the atmospheric condition, resulting in corrosion rates directly comparable with those experienced at the coastal splash-zone site.

Citation

SHAW, D. 1979. Corrosion of steel in the marine splash-zone. Robert Gordon's Institute of Technology, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-1993224

• Thesis Type: Thesis
• Deposit Date: Oct 14, 2024
• Publicly Available Date: Oct 15, 2024
• DOI: https://doi.org/10.48526/rgu-wt-1993224
• Public URL: https://rgu-repository.worktribe.com/output/1993224
• Award Date: May 31, 1979