51314-4249-Contact and Optical Profilometry for Quantitative Characterisation of Pitting Corrosion

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Product Number: 51314-4249-SG

ISBN: 4249 2014 CP

Author: Trevor Hughes

Publication Date: 2014

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This study compares methods based on contact profilometry and optical profilometry (3D microscopy) in terms of their ability to provide a quantitative characterisation of pitting corrosion on a series of test coupons prepared from duplex steel (2205) oilfield casing. The test coupons were exposed to a series of oilfield acidizing formulations for 3 hours at 78C. The formulations contained 4mol/L hydrochloric acid and increasing concentrations of the inhibitor naphthylmethylquinolinium chloride (4–30mmol/L). A sub-set of the fluids also contained potassium iodide. After exposure we observed a systematic decrease in coupon weight loss with increasing inhibitor dosage. With increasing weight loss we observed an increasing extent of pitting corrosion which was described by a visual pitting index (PI) ranging from PI=0 (no pitting) to PI=9 (severe and extensive pitting). The overall aim was to develop a new analytical method to categorize the pitting behaviour objectively.The contact profilometer was used to generate ten 13mm line profiles per test coupon five on the outer and five on the inner face. The 3D microscope was used to collect digital images of four areas (each 0.75mm by 15mm) per test coupon two on the outer and two on the inner face. A range of different surface roughness parameters were calculated from the contact line profiles and digital images. Additional parameters were computed from the images including pit density pit dimensions (depth and diameter) and pit volume per unit surface area. We discuss observed changes in roughness parameters and additional parameters with increasing extent of pitting corrosion. The results indicate that the extent of pitting corrosion is most sensitively described by the 3D parameter pit volume per unit surface area which differentiates subtle changes in the PI range 0 to 2. In addition the results show that for this particular metal type and corrosive fluid conditions pit depths are 22-35 micron when the PI is 1 to 2 increasing to 85 micron when the PI is 4 to 5. With increasing weight loss and extent of pitting corrosion the surfaces evolve to an etched texture which reflects preferential dissolution of the ferrite matrix relative to austenite. This is supported by analyses of the elemental composition of the surfaces using X-ray fluorescence (XRF) spectrometry.The methods developed during this study can be broadly applied to the quantitative characterisation of pitting corrosion of other metal types exposed to different corrosive fluid conditions.