Effect of Temperature and Pressure on the Morphology of ... - Corrosion

0 downloads 0 Views 168KB Size Report
Carbon dioxide (CO2) corrosion product scales of three kinds of oil well steel were obtained from simulated tests in autoclaves at varying temperatures and ...
CORROSION SCIENCE SECTION

Effect of Temperature and Pressure on the Morphology of Carbon Dioxide Corrosion Scales G. Lin,‡,*,** M. Zheng,** Z. Bai,*,** and X. Zhao**

ABSTRACT

INTRODUCTION

Carbon dioxide (CO2) corrosion product scales of three kinds of oil well steel were obtained from simulated tests in autoclaves at varying temperatures and pressures. The effect of temperature and CO2 partial pressure on the thickness and grain size of the scales was studied with the help of scanning electron microscopy (SEM). The results show that the cross sections of the corrosion product scales of the three kinds of steel are all of a double-layer structure. The variation manner of the scale thickness of the three kinds of steel with the temperature are quite similar, and the maximum scale thickness appears at 120°C; however, the temperature for the minimum of the scale thickness is variable. The curves of the grain sizes with the temperature display a peak and a valley for the three kinds of steel. The variation of scale thickness with CO2 partial pressure for the three kinds of steel is also quite similar, in which the maximum scale thickness appears at the CO2 partial pressure of 6.89 MPa and then decreases rapidly when the CO2 partial pressure increases. The variation of grain size with CO2 partial pressure shows two peaks and a valley for each kind of steel.

Carbon dioxide (CO2) corrosion has become one of the most serious corrosion types of oil well tubular goods, which has induced a large amount of economic loss and calamity. Many investigations show that ferrous carbonate (FeCO3) is the basic composition in the corrosion product scale on the steel substrates in saturated and nonsaturated CO2 solution containing oil. Whether or not the scale can protect the substrate from CO2 corrosion depends on its physical characteristics.1 The complete, close-grained, and more adhesive protecting scale can decrease the corrosion rate. The physical characteristic of the scale is closely related to its forming conditions. Temperature and CO2 pressure are two of the important external factors in the CO2 corrosion process. Schmitt2 considered that there is an intrinsic change in the kinetics in the CO2 corrosion process around 60°C. Many investigators2-5 also observed the following: —Uniform corrosion takes place on the surface of a ferroalloy below 60°C. —The corrosion product scale is a noncompact, soft, and nonadhesive ferric carbonate. —The surface of the substrate is smooth. —There is a transitional range between 60°C and 110°C, in which the ferroalloy could produce the scale with a certain protecting property and evident local corrosion. —Around 110°C, the uniform corrosion rate is high, local corrosion is serious, and the scale contains thick crystals.

KEY WORDS: carbon dioxide corrosion, corrosion scale, grain size, pressure, scale thickness, temperature

Submitted for publication March 2005; in revised form, November 2005. ‡ Corresponding author. E-mail: [email protected]. * School of Material Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China. ** Tubular Goods Research Center of CNPC, Xi’an 710065, China.

CORROSION—Vol. 62, No. 6

0010-9312/06/000091/$5.00+$0.50/0 © 2006, NACE International

501

CORROSION SCIENCE SECTION

TABLE 1 Chemical Composition of the Three Steels (wt%) Steel Grade

C

Si

Mn

P

S

Cr

Mo

Ni

Ti

Cu

N80 P110 J55

0.24 0.26 0.19

0.22 0.20 0.31

1.19 1.40 1.39

0.013 0.009 0.014

0.004 0.003 0.004

0.036 0.150 0.190

0.021 0.010 0.092

0.028 0.012 0.017

0.011 0.030 0.040