Near-neutral pH corrosion of mill-scaled X-65 pipeline steel with paint primer

The corrosion behaviour of mill-scaled X65 pipeline steel with and without a primer layer was studied in a simulated near-neutral pH soil solution.Results revealed a three-stage corrosion process of the mill-scaled pipeline steel surface.The first stage included an initial preferential dissolution of goethite(γ-FeOOH)and lepidocrocite(?-FeOOH)in mill scale.The second stage was marked by enhanced localized corrosion and pit-formation because of either galvanic corrosion or acidic dissolution in areas enclosed by mill scale.The final stage was general corrosion after the mill scale flaked off the steel surface.When the primer layer was applied,localized corrosion was significantly enhanced on the steel surface and persisted for an extended period as compared to the mill-scaled condition.The precipitation of siderite(FeCO3)was observed at flawed locations of mill scale,although the bulk chemistry is not favorable for its formation on the steel surface free of mill scale.The local precipitation of siderite formed a capped mill scale enclosure where localized corrosion can be further enhanced.