Spectral Analysis Of CO2 Corrosion Product Scales On 13Cr Tubing Steel
Authors: Lin Guanfa , Feng Yaorong , Bai Zhenquan , Xiang Jianmin 
Published: May 2016

Abstract
CO2 corrosion product scales formed on 13Cr tubing steel in autoclave and in the simulated corrosion environment of oil field are investigated in the paper. The surface and cross-section profiles of the scales were observed by scanning electron microscopy (SEM), the chemical compositions of the scales were analyzed using energy dispersion analyzer of X-ray (EDAX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to confirm the corrosion mechanism of the 13Cr steel in the simulated CO2 corrosion environment. The results show that the corrosion scales are formed by the way of fashion corrosion, consist mainly of four elements, i.e. Fe, Cr, C and O, and with a double-layer structure, in which the surface layer is constituted of bulky and incompact crystals of FeCO3, and the inner layer is composed of compact fine FeCO3 crystals and amorphous Cr(OH)3. Because of the characteristics of compactness and ionic permeating selectivity of the inner layer of the corrosion product scales, 13Cr steel is more resistant in CO2 corrosion environment.

1. Introduction
The carbon dioxide corrosion is one of the main types of oil tube damage and failure in environment of oil and gas exploitation [1]. The carbon steel and low alloy steel could be resistant to the CO2 corrosion under low temperature and common pressure condition and can also meet the requirement specified in relative standard [2,3]. With increasing the depth of the well the temperature and pressure get higher and higher, the corrosion condition becomes more rigorous, which leads to that some advanced steels with good corrosion resistance such as some stainless (304L, 316L, et al) and high Cr steels (HP13Cr type, 18Cr type, 22Cr type, 25Cr type or SAF 2205, et al) have been used in some oil and gas fields [4–7], in spite of the much higher exploiting cost.
Many researches [7–10] showed that the corrosion resistance and the price of common 13Cr steel is between that of common carbon steel and advanced corrosion resistance steel, so it is necessary to research the corrosion resistance characteristic of common 13Cr steel in the corrosion environment with higher temperature and pressure, and is helpful for petroleum industry to choose the reasonable material used in the corrosion environment and to increase the commercial benefit of the oil fields.
For evaluating the corrosion resistance character of common 13Cr steel, the CO2 corrosion product scales on common 13Cr steel were prepared in autoclave by simulating more serious corrosion environment of oil field, and their chemical compositions were confirmed using spectral analysis associated with analyzing the profile characteristics to infer the forming mechanism of the corrosion scale.

2. Experimental
The tested material is 13Cr steel with the chemical composition listed in Tab. 1.
The samples were machined into arc slices of F72.0mm×11.0mm×3.0mm (their central angles are all 60°), and the investigated surfaces were polished with sand papers from 120# to 1000#, then put in acetone for removing oil, and dried. The other surfaces were sealed up with silicon rubber NQ-704. The samples were fixed on the outer circumferential surface of a discal clamp with the silicon rubber, placed in a desiccator for 24h solidified.