Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: In this study,two sandblasting textures,namely,quartz and high-chromium (Cr) stainless steel sands,were used for sandblasting of the internal surface L80-13Cr tubing and casing.The contrastive analysis on the properties of the pipes before and after abrasive blasting was conducted,including the metallographic phase detection of the internal surface,simulating accelerated corrosion tests,and residual stress tests for the pipe after abrasive blasting.Based on the analysis results,it is explained why the API standard requires that no scale appears on the internal surface of L80-13Cr and iron contamination when internal sandblasting media are applied.The results show that abrasive blasting can effectively remove scales on the internal surface of L80-13Cr tubing and casing and improve corrosion resistance.The abrasive blasting process does not produce obvious residual stress on the internal surface of the tubing and casing.Also,the de-rusting effect of stainless sand is better than that of quartz sand,and the former does not produce iron contamination.

Key words: L80-13Cr; tubing and casing; abrasive blasting; corrosion

1 Introduction

Recently,there has been an in-depth growth of energy exploitation,oil and gas blocks,which have been increasingly developed with severe corrosion conditions,and the demand for tubing and casing with special material and good performance of corrosion resistance has gradually increased.As a CO2-corrosion-resistant tubing and casing product,L80-13Cr has been widely used in offshore oil and gas fields.With the development of Baosteel’s manufacturing technique,L80-13Cr has gradually shifted from only supplying plain-end pipes to foreign countries to the production of finished tub-ing and casing products.

According to the American Petroleum Institute (API) 5CT standard,the internal surface of L80-13Cr products should be without scales after the final heat treatment.For PSL2 products and above,the internal surface is processed with sandblasting by applying a medium that does not contaminate the surface iron,and the performance of the surface should meet the requirements of Sa 2-1/2 in ISO 8501-1 after the treatment.However,the cause in the standard has been clarified.Considering the simulation of on-site use,herein,we selected high-Cr stainless steel and quartz sand for abrasive blasting toward the internal surface of L80-13Cr pipes,in contrast to the pipe not processed by sandblasting.We examined the change of features before and after removing the scales on the internal surface of L80-13Cr tubing and casing to provide technical support for improving the manufacturing technique of finished L80-13Cr tubing and casing.

2 Test method

2.1 Sample preparation

A qualifiedφ73.02 mm×5.51 mm L80-13Cr pipe was selected as a trial raw material and was subjected to 960 ℃ oil quenching and 680 ℃ tempering heat treatment.Rust was removed from the internal surface by conventional abrasive blast-ing of quartz and high-Cr stainless steel alloy sands.The rust removal effect meets Sa 2-1/2 requirements of ISO 8501-1.Pipe samples that were processed by sandblasting were cut out for comparison.

The surface of the internal wall of the pipe was observed,and the samples were acidified by cutting the pipes in various states to 20 mm×50 mm test blocks.

The sample for residual stress tests was prepared through cutting and sewing.The length of the test pipe was twice the external diameter.

2.2 Test method

A stereoscopic microscope and a scanning elec-tron microscope (SEM) were used to observe the morphology of the internal surface of the original and acidified pipes.Acidizing fluid employed in simu-lated oil field working environment was adopted.The composition of the solution is 10%HCl+2%HF+3%HAc+corrosion inhibitor.The test was conducted at 85 ℃ for 6 h.

The residual stress of the pipe was calculated using Equation (1) after measuring the cut and sewed sample[1]:

σT=(E·T)(1/DO-1/Df)/(1-υ2)

(1)

where,σTis residual stress,MPa;Eis the elastic modulus,E=205 950 MPa;Tis the wall thickness (the average value of 8 points on the top and bottom),mm;DOis the outer diameter (the average value of the outer diameters of 4 points on the top and bottom),mm;Dfis the outer diameter after incision (the average value of the outer diameters of 4 points on the top and bottom,and the incision is staggered with the measuring point),mm;andυis the Poisson’s ratio,υ=0.29.

3 Test results

3.1 Surface of the internal wall before and after abrasive blasting

Fig.1 shows the morphology of the internal wall’s surface of the pipes not sandblasted and those sand-blasted using quartz and high-Cr alloy sands observed using a stereoscopic microscope.The pipe without sandblasting is numbered 1#,that sandblasted using quartz sand is numbered 2#,and that sandblasted using high-Cr alloy sand is numbered 3#.There were scales on the surface of the internal wall of the pipe without sandblasting.The scales peeled off in some areas.Since the scales are composed of brittle oxide,there were numerous microscopic cracks on the scales that did not peel off.There was no scale on the internal wall of the pipe subjected to abrasive blasting using quartz and alloy sands.

Fig.1 Morphology of the internal wall of the pipes observed using a stereoscopic microscope

Fig.2 shows the SEM images of the internal wall of pipes in three states and the analytical results of the energy spectrum.The internal wall of the pipe without sandblasting was covered with oxide,and energy spectrum analysis revealed that the oxide layer consists of oxides,namely,Cr and Fe.After sandblasting using quartz sand,the internal wall showed poor conductivity,and the non-ideal imag-ing effect was covered by broken quartz sand powder during the abrasive blasting process.Energy spectrum analysis also revealed more severe surface contamination with higher content of the C element.Surface constituents of the alloy sand after abrasive blasting were mainly the matrix.A small amount of C and O elements was generated by the contamination of the sample’s surface.Also,cutting marks were obser-ved on the surface of the alloy sand after abrasive blast-ing,attributed to alloy sand on the surface of the pipe.

Fig.2 SEM images of the internal wall of the pipes and analytical results of the energy spectrum

3.2 Analytical results of the internal wall’s surface quality after acid corrosion test

Fig.3 shows the morphology of the internal wall of the pipes observed using a stereoscopic micro-scope after acidification in three states.The pipe 1#shows more erosion pits on the surface,shallow strips visible in almost all areas in the internal surface,and longer and deep grooves in several areas.After sandblasting the corrosion on the in-ternal wall,the general morphology shows uniform corrosion,and no apparent erosion pit is observed.There are yellow substances distributed on the dents of the internal wall’s surface of the quartz sand sand-blasted through acidification.

Fig.3 Morphology of internal wall of the pipes after acidification in three states,as observed using a stereoscopic microscope

Fig.4 shows the SEM images of the internal wall of the pipes after acidification in three states.The grooves of the internal surface of pipe 1#were sub-jected to corrosion and pitting.Meanwhile,the lines revealed by the stereoscopic microscope were com-posed of several small erosion pits.Such lines are attributed to microscopic cracks in the scale.During the etching test,the corrosive medium was prepared by preferentially infiltrating from microscopic cracks into the juncture between the scale and the matrix.After abrasive blasting,the SEM image showed no distinct corrosion on the internal wall of the pipe,whereas on the internal wall of pipe 2#,although corrosion occurred while soaked in the acidizing fluid,there was a small amount of residue of quartz sand,and cutting marks were left on the surface after the abrasive blasting of alloy sand.

Fig.4 SEM images of the internal wall of the pipes after acidification in three states

3.3 Analytical results of residual stress before and after abrasive blasting

Pipes acidified in each state were tested with two parallel samples on one pipe,and the test results are shown in Table 1.The residual stress of the pipe sandblasted using quartz sand is slightly lower than that of the pipe without sandblasting and that sand-blasted using alloy sands but in the same order of magnitude as that of the pipes in the three states.Since there is a discrepancy among the residual stress based on the status of each pipe,the test results show little difference in the residual stress before and after abrasive blasting.

Table 1 Residual stress of the pipe MPa

4 Analysis and discussion

During the rolling and thermal treatment,scales were produced on the internal walls of the L80-13Cr tubing and casing.The scales unevenly covered the surface,and the moisture absorption was extremely asymmetrical,which aggravates the attachment of mud in the actual use.Meanwhile,there was a thin depleted chromium layer under the scales,aggrav-ating pitting and under deposit corrosion.Fig.5 shows the morphology of the internal wall of a 13Cr pipe used in an oil field.Crack initiation and expan-sion were observed at the residue of the original scale.Also,grooves and erosion pits were produced near the original scale during simulating the acidifi-cation,which explains why the API 5CT standard requires that the scale should not exist on the internal wall of L80-13Cr products and the ISO 13680 standard that requires the same for high alloy products,such as super 13Cr.Considering the actual de-rusting effect,high-Cr stainless steel alloy sands can remove oxides on the internal wall more thoroughly,making the internal surface of tubing and casing have a uniform and accordant matrix texture.

Fig.5 Morphology of the internal wall of 13Cr pipe that is out of work in an oil field

A previous study[2]showed that scratches were generated when 13Cr stainless steel was scraped by carbon steel.It cannot tarnish at the scratches,which is the so-called iron contamination effect as the standard stated.Such tarnishing is not the corrosion of 13Cr but the carbon steel remaining on the surface of the material when 13Cr stainless steel is scraped,which erodes in corrosive environments.Corrosive products contaminate the surface of the material.The phenomenon is based on the fact that the corrosion potential of carbon steel is signifi-cantly lower than that of 13Cr stainless steel.The tarnishing preferentially occurs when carbon steel remains on the surface of 13Cr stainless steel,in-dicating that 13Cr stainless steel corrodes by itself.Due to the larger potential difference between the two,the surface of 13Cr stainless steel with residues of carbon steel contamination is likely to become a breakthrough of electrochemical corrosion in cor-rosive environments when getting bumped and damaged.

As shown in Fig.6,in practice,after the abrasive blasting of the internal surface of L80-13Cr tubing and casing by applying ordinary steel sand,such as cast steel,a layer of floating rust appears on the internal surface of the pipe shortly,indicating that cast steel remaining on the internal wall gets tarnished,confirming the above analysis.The high-Cr alloy sands used in this study are martensite stainless steel containing about 30% Cr and 2% C,equivalent to the stainless steel contacting each other during abrasive blasting.Based on both com-parative corrosion potential,corrosion of the poten-tial difference does not occur like that in ordinary carbon steel sandblasting materials,which is the so-called carbon steel contamination.The result of this study also shows that no abnormal signs,such as grooves and pitting,are observed when the internal surface of the alloy is sandblasted using nonmetallic quartz and high-Cr stainless steel alloy sands.

Fig.6 Morphology of the internal wall of L80-13Cr after cast steel sandblasting

5 Conclusions

In this study,we observed the morphology of the internal wall of L80-13Cr pipes processed through different sandblasting media and those without sandblasting,conducted etching tests,and analyzed the residual stress.Combining the analysis of API 5CT product standard,the following conclusions are drawn:

(1) During the production of L80-13Cr tubing and casing,scales formed on the internal surface aggravate pitting and under deposit corrosion in subsequent services in the oil and gas field.

(2) Abrasive blasting of internal surfaces can effectively remove scales on the internal surface of L80-13Cr tubing and casing and improve its cor-rosion resistance.

(3) The abrasive blasting technique does not pro-duce evident residual stress on the internal surface of tubing and casing products;hence,it does not aggravate the risk of corrosion cracking of the products in subsequent processes in the oil and gas field.

(4) High-Cr stainless steel-alloy sands show a better effect in removing scales from the internal surface of L80-13Cr than quartz sands.In addition,there is no potential corrosion on the L80-13Cr pipe,which is called surface iron contamination.