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ABSTRACT Supermartensitic stainless steel (SMSS) weldments are found to be prone to pitting corro- sion at ambient temperature in simulated formation waters containing traces of H2S. The present work has been undertaken to clarify how the occurrence of low temperature pitting corrosion de- pends on the presence of H2S and the surface condition of the weldment. It appears that the as- welded SMSS root surfaces are not fully passivated when they are tested at ambient temperatures and that the main effect of H2S is to hinder repassivation and to accelerate corrosion occurring in the oxidized region close to the fusion line. The increased corrosion susceptibility seems to be a result of high temperature oxidation during welding modifying the surface condition. Corrosion tests have been used to simulate repassivation of "as-welded" SMSS root surfaces during pre- exposure to various oxidizing electrolytes. Some practical implications with respect to qualifica- tion of SMSS weldments are discussed. Based on these findings, it is recommended that a com- mon pre-exposure in an oxidizing electrolyte is carried out before the samples are subjected to corrosion testing. The test procedure should reflect the modification of the SMSS weldments tak- ing place prior to exposure to H2S. INTRODUCTION A major concern in the qualification and quality control of SMSS weldments for unproc- essed well stream environment containing CO2 and traces of HzS (slightly sour conditions) has been the resistance to environmentally cracking (EC). The European Federation of Corrosion pub- lication No. 17 (EFC 17) 1 has been adopted as a general guideline for the assessment of corrosion resistant alloys (CRAs) in sour service. EFC 17 is mainly concemed with EC, but stresses the im- portance of verifying an adequate general and localized corrosion resistance prior to performing EC tests 1. As guidance, EFC 17 points out that the worst conditions for general and localized cor- rosion are those normally associated with the maximum service temperature. Thus, corrosion tests should be performed under such severe conditions. The qualification of SMSS weldments for the Asgard and Gullfaks Satellite field devel- opment projects 2'3 was accomplished in accordance with these principles I with HzS partial pres- sures of 4-8mbar (i.e. Gullfaks) and 40mbar (i.e. Asgard). Consequently, pitting tests were carried out at anticipated maximum service temperatures of 90-140°C, demonstrating adequate properties of the actual weldments 2'3. Since pitting corrosion is not a major concern under the prevailing cir- cumstances this has cleared the way for EC tests in the form of constant load (CL) and four-point bend (FPB) testing. EC tests soon showed, however, that the SMSS weldments were sensitive to general corrosion (experienced in simulated condensed water, 8mbar HzS, pH 3.5-3.8, low [C1-])4,5 and to pitting corrosion (experienced in simulated formation water, 40mbar HzS, pH 4.5- 6, high [C1-])3 at ambient rather than at elevated temperatures. This contradicts the anticipated behavior of CRAs, thus indicating a subtle, new corrosion mechanism that most probably involves HzS. Moreover, in agreement with the martensitic microstructure and with the nature of SSC, cracks occurred within regions characterized by active dissolution at ambient temperature, while no cracks were observed at the elevated temperatures. The aim of the present paper is to clarify why pitting occurs in SMSS weldments at ambi- ent rather than at elevated temperatures and how this can be prevented. Moreover, results from a series of corrosion tests designed to examine the repassivation of "as-welded" root surfaces of SMSS weldments are included. Finally, some pract
- Europe > Norway (0.29)
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- Research Report (0.69)
- Overview > Innovation (0.40)
- Europe > Norway > Norwegian Sea > Halten Terrace > Block 6507/11 > Åsgard Field > Åre Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > Block 6507/11 > Åsgard Field > Tofte Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > Block 6507/11 > Åsgard Field > Tilje Formation (0.99)
- (44 more...)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
ABSTRACT The hardness values of weldments of supermartensitic stainless steels will exceed generally acceptable limits for sour service and qualification testing with simulation of the actual conditions has to be done. Use of international guidelines, e.g. the EFC-17 document, has shown that there is a need to specify more details for the testing. The high temperature oxidation during welding will weaken the pitting resistance for SMSS exposed to slightly sour environment. This will influence the initiation of pitting corrosion and thus the resistance to SSC. The buffer strength in a simulated environment may influence the results and should thus be selected to avoid artificial effects only seen in the laboratory. pH adjustment of a simulated buffer must be done with the test gas in the system. It is important to check that there has not been any significant change in the pH during testing. In high pressure testing, the pH should be checked prior to testing and after the testing with 1 bar CO2. When stressing 4PB specimens with the root intact, the global deflection to be used must be checked and calibrated against strain gauge measurements in the HAZ prior to the corrosion testing. In case of significant low temperature creep, a 4PB specimen will experience stress relaxation. If testing of an intact root surface is required, no standard test methodology is available if this relaxation should be avoided. INTRODUCTION Weldable supermartensitic stainless steels (SMSS) have been used in several subsea flowlines for transport of unprocessed well stream containing CO2 and small amounts of H2S. Most pipelines have up to now been welded with a super duplex stainless steel (SD) consumable. There has been a development of matching consumables that now are available on the market. The general requirement for martensitic stainless steels used in sour environment is to limit the hardness to 22 HRC1. Due to the inherent structure of the material, it is not possible to limit the hardness in the weld zone in a SMSS material to this value. Even with a post weld heat treatment (PWHT), the hardness in the heat affected zone (HAZ) will typically be higher than 300 HV, which corresponds to approximately 30 HRC. When using a matching consumable, the hardness in the weld metal will be above or in the same order as for the HAZ. Typical hardness distributions for weldments made by a matching consumable is shown in Figure 1. In order to use this material in slightly sour environments, fitness for purpose testing is required. Oil companies, pipe suppliers and pipe laying contractors have performed such testing for specific projects. There are a variety of publications on this subject in the open literature where different grades and weldments have been tested in different environments. Most testing has been done according to the procedure defined in EFC Publication No 172. It has been reported some special effects observed during corrosion testing3,4 , which has led to a discussion on the relevance of the actual test methods and test procedures. The intension of this paper is to discuss some of these aspects.
Intergranular Corrosion/Cracking of Weldable 13%Cr Steel at Elevated Temperature
Rogne, Trond (SINTEF Materials Technology) | Lange, Hans I. (SINTEF Materials Technology) | Svenning, Morten (SINTEF Materials Technology) | Aldstedt, Synnove (SINTEF Materials Technology) | Ladanova, Elena (NTNU) | Solberg, Jan Ketil (NTNU) | Olsen, Stein (BP Upstream Technology Group) | Howard, Roger (BP Upstream Technology Group) | Leturno, Richard E. (BP Upstream Technology Group)
ABSTRACT Corrosion and mechanical tests were performed on two large diameter seam welded 13%Cr stainless steel pipes (1.5 Mo and 2.5 Mo). The testing included seam welds, mechanised gas metal arc welded (GMAW) girth welds and base materials. No evidence of sulphide stress corrosion (SSC), local pitting or crevice corrosion was observed in either of the two materials or the welds when tested at 27°C in formation water and in condensed water at 20bar CO2 and 2mbar H2S. No pitting or crevice corrosion was observed at 110°C. However, by 4-point-bend testing, corrosion/cracking occurred in the girth welded samples in both environments at l l0°C. The corrosion/cracking initiated at the region of the root fusion line and propagated in the high temperature Heat affected Zone (HAZ). Initial and secondary cracking followed the prior austenite grain boundaries. The mechanism is typical intergranular corrosion combined with stress corrosion cracking. Transmission Electron Microscopy(TEM) investigation has documented M23C6 precipitates at the prior autenite grain boundaries. Post weld heat treatment (PWHT) at 650 °C for minutes seems to have a beneficial effect on the corrosion/cracking behaviour. The super duplex weld metal used for the girth welds overmatched the yield strength of the parent materials at room temperature but undermatched it at 110 °C. Compared to the value at ambient temperature, an increase in yield strength of approximately 10% was recorded for the 13%Cr pipe materials at 110°C probably due to the present of retained austenite. A decrease of approximately 10-15 % in the yield strength of the duplex weld metals was recorded at 110°C. INTRODUCTION Atlantic Richfield Company (ARCO), now BP, appointed SINTEF Materials Technology to evaluate candidate weldable 13%Cr martensitic stainless steel linepipe materials for the Tangguh gas field development. In addition to the 13%Cr alloys, a number of other corrosion resistant alloys (CRA) and internally clad products were under consideration for use as pipe line materials. However, significant cost savings can be achieved in some development scenarios by the use of the weldable 13%Cr alloys particularly when relatively large diameter pipe lines are required. Furthermore, appropriate use of CRA pipelines can significantly enhance the reliability of pipeline systems. Reliability is of paramount importance for the Tangguh pipelines since the associated LNG plant will demand a high level of availability of gas from the offshore Gas Production Facilities. The present study undertook a battery of mechanical tests and corrosion evaluations on two large diameter seam welded 13%Cr stainless steel (1.5Mo and 2.5Mo) pipes. The testing included seam welds, mechanised GMAW girth welds and the base materials. The mechanical test programme included tensile, charpy and CTOD evaluation. Microstructural assessments and hardness testing were also undertaken. The corrosion studies involved SSC testing, and pitting and crevice corrosion evaluation at ambient and service temperature. Seawater corrosion and cathodic protection testing was also performed. Some additional testing were performed with funding from Statoil in Norway to obtain a better understanding of the corrosion that where uncovered.
- Europe > Norway (0.35)
- Asia > Indonesia > West Papua (0.24)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)