THE EFFECT OF SOLUTION ANNEALING ON THE CORROSION RESISTANCE OF A WELDED 0.36%C STEEL IN CHLORIDE ENVIRONMENT

ABSTRACT

Medium carbon steel is highly used in the petroleum and chemical industry, and solution annealing has been demonstrated in other kind of steel to be very effective in controlling corrosion. This thesis intends to investigate the effect of solution annealing on the corrosion resistance of a welded 0.36%C steel in chloride environment. The specimens were machined into weldable specifications and welded using the technique of shielded metal arc welding (SMAW). The samples were then solution annealed to determine optimum holding time and temperature. On establishing the optimum time and temperature, samples were now solution annealed at these optimum conditions to determine associated microstructural properties. For the microstructural analysis, the specimens were subjected to optical microscopy (OM) and energy dispersive X-ray spectrometry (EDX). Also hardness tests were conducted using Vickers microhardness testing (VMT) as prescribed by American Standard Testing Method (ASTM) A384/89 standard. The corrosion rate was determined using gravimetric analysis in 1.0M NaCl environment at intervals of 20 days for the period of 100 days. Potentiostatic measurement method was also used to determine corrosion rate, corrosion potential, corrosion current from tafel slope, polarization resistance, open circuit potential, electrochemical noise, and electrochemical impedance spectroscopy respectively. The result obtained from the solution annealing process established the optimum annealing temperature and time for the 0.36%C steel as 990 oC and 2.0 hours respectively. The hardness results obtained showed that the solution annealed samples have higher hardness property than the control samples at the welded zone, heat affected zone and the parent metal. When exposed to corrosion, the obtained corrosion rate from gravimetric analysis and Potentiostatic measurement method showed that the solution annealed samples were found to be more noble in the chloride environment than the control samples. The nobility of the samples was found to decrease across the welded zones. The solution annealed parent metal was found to have a higher polarization resistance of 786.061 Ω.cm2 and with a lower corrosion rate of 0.591 mm/yr, while the control parent metal was found to have a lower polarization resistance of 269.867 Ω.cm2 and with a higher corrosion rate of 1.673 mm/yr. The solution annealed heat affected zone was found to have a higher polarization resistance of 180.452 Ω.cm2 and with a lower corrosion rate of 1.017 mm/yr, while the control heat affected zone was found to have a lower polarization resistance of 61.263 Ω.cm2 and with a higher corrosion rate of 3.143 mm/yr. The solution annealed welded zone was found to have a higher polarization resistance of 54.483 Ω.cm2 and with a lower corrosion rate of 3.207 mm/yr, while the control welded zone was found to have a lower polarization resistance of 41.133 Ω.cm2 and with a higher corrosion rate of 4.832 mm/yr. The solution annealed samples were not prone to pitting corrosion and less susceptible to uniform corrosion compared to the control samples that were found to be prone to pitting corrosion and more susceptible to uniform corrosion. The susceptibility of the samples was found to increase from the parent metal, to the heat affected zone with the welded zone as the most. This study, therefore shows that the solution annealing heat treatment was effective toward corrosion protection of the investigated steel in chloride environment. Welded medium carbon steel used for oil and gas pipelines will therefore fare better under the influence of solution annealing in seawater in Nigeria