This paper presents the effect of the deposition of sand and actual field sludge on the corrosion mechanism of carbon steel in sour environments. Under-deposit corrosion (UDC) of carbon steel was studied underneath the sand deposit and the field collected sludge deposit in a simulated sour environment by means of linear polarization resistance (LPR), coupled multi-electrode array system (CMAS) and weight-loss coupons.
The results showed that when actual field sludge was deposited on the carbon steel surface, significant general and localized corrosion rates were observed. While in the sand deposit test, the general and localized corrosion rates were found to be very low, even lower than the no deposit test. No UDC was observed on coupons tested under the sand deposit, meaning that sand inhibits corrosion of carbon steel in a sour environment.
Under-deposit corrosion (UDC) in oil and gas pipelines is a treacherous attack that initiates at a break in the protective corrosion product layer (e.g., iron sulfide (FeS)) or underneath of the sludge deposits. The pipeline sludge deposit is usually comprised of sand, asphaltenes, FeS, iron carbonate and barium sulfate and other debris that is saturated with brine, and frequently contains high concentrations of chloride ion and dissolved acid gases (carbon dioxide (CO2) and hydrogen sulfide (H2S)). The corrosivity of the system can be further exacerbated by bacterial growth and the co-deposition of wax or paraffin materials onto the surface deposits. It has frequently been cited as responsible for high corrosion rates and loss of containment during operations [1-4].
It is reported in the literature [5, 6] that the UDC results in localized corrosion, formation of pits and mega-attacks. A theory was postulated [7, 8] that localized corrosion under deposits is the result of galvanic corrosion between steel and sludge deposits. Similar opinions were reported elsewhere [9, 10]. The exact mechanism of this phenomenon is dependent upon the local operating conditions, thickness and type of the deposits.