Development of new method to characterize the durability of stainless steels to crevice attack in natural and treated seawaters (CREVCORR)

Projektinformationen

CREVCORR

ID Finanzhilfevereinbarung: G5RD-CT-2000-00139

Projekt abgeschlossen

Startdatum 15 April 2000

Enddatum 14 Oktober 2003

Finanziert unter

• Programme for research technological development and demonstration on "Competitive and sustainable growth 1998-2002"

Gesamtkosten

€ 1 312 747,00

EU-Beitrag

€ 649 948,00

Koordiniert durch

DET NORSKE VERITAS A/S
Norway

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Verwertbare Ergebnisse

Report specifying artificial seawater with biocapacity and discussing anaerobic biofilm/synergistic effects

Aerobic bacteria settled on the stainless steel surface fraction freely exposed to natural seawater and SRB bacteria active under shielded areas act in synergistic way as promoters of crevice corrosion onset, being the cathodic reaction(s) accelerated by the aerobic biofilm and the anodic passivation locally decreased by the anaerobic biofilm. In laboratory tests, the effect of aerobic bacteria can be simulated by adding glucose oxidase and glucose to sterile seawater whereas the effect of anaerobic bacteria can be simulated by adding Na2S and HCl to sterile de-aerated seawater. A total sulphide concentration of about 400 ppm and a pH in between 6.7 to 7 is suggested. The settlement of aerobic bacteria on the SS surface fraction freely exposed to natural seawater and the presence of active SRB bacteria under shielded areas act in synergistic way as promoters of crevice corrosion onset, being the cathodic reaction(s) accelerated by the aerobic biofilm and the anodic resistance locally decreased by the anaerobic biofilm. The report presents a new synthetic seawater with biocapacity that can simulate natural seawaters at ambient temperature.

Specification of natural and treated seawaters

Natural and treated seawaters which are relevant for qualification testing of stainless steels from an industrial point of view have been defined. Four different kinds of water have been specified. The first one simulates natural seawater at ambient temperature, while the second type simulates natural seawater at elevated temperatures. The third water type simulates seawater cooling waters and the fourth type simulates downhole brine or injection water.

Report on calibration of the participating laboratories

The report describes the results of an intercomparison test of the laboratories participating in the Crevcorr-project. The Critical Crevice Temperature (CCT) of the 2205 duplex and the 254SMO super austenitic stainless steel was determined by several laboratories according to a well defined procedure. Rectungal coupons were equipped with multi-crevice-washers and exposed in a ferric chloride solution at three different temperatures, after which the resulting attack was noted. Amongst the possible criteria, it would appear that a maximum depth of attack equalling or exceeding 25 µm allows a relatively accurate determination of CCT, even though significant scatter exists between individual depth results from different laboratories and in some cases between duplicate tests.

Electrochemical simulation of the corrosivity of the different industrial marine waters

The corrosivity found in different types of industrial marine waters can be simulated electrochemically when testing stainless steels. One important aspect in this respect is how to make electrical connection to the specimen. The work performed verifies that the most convenient way to perform this is to allow the test specimen to hang in a titanium wire. The work presents the electrochemical representation of the biofilm effect found in natural seawaters, and the oxidation capacity found in chorinated seawaters. The work presents the results from a small round robin test, indicating that the proposed procedures to electrochemically simulate the characteristic of different seawaters, seems to reflect the behaviour in real waters well.

Verification of the reliability and reproducibility of the proposed test procedures

In the Crevcorr-project two of the tasks were aimed to develop new test techniques or methods for evaluating the crevice corrosion susceptibility of stainless steels. Based on these tests a thorough description of testing procedures was made and laboratories inside the project consortium and worldwide were invited to participate. A total of 19 laboratories volunteered to participate providing all test samples, necessary equipment and procedures were delivered by the consortium. Two round robin tests were undertaken. The main objective of the round robin tests was to verify the reliability and reproducibility of new developed test methods to characterize the crevice corrosion resistance of stainless steels. A second objective has been to contribute to establishing new guidelines for qualifying stainless steels for use in natural and treated seawater. Based on the reported test results from a round robin test performed both on natural seawater and in synthetic biochemical seawater it has been shown that the general description of the test method with respect to crevice assembly was simple and easy to follow. The crevice assembly was robust and small deviations were reported. The method was reliable and reproducible.

Specifications on test procedures and waters to be used in the round robin test

The test specification contain 4 attachments: one for the general test procedure, one for the natural seawater test, one for the synthetic biochemical seawater test and one testreport scheme. The test procedures describe in detail how to perform the crevice corrosion testing and how to make the synthetic seawater with biochemical capacity. The procedure contains a detailed description of the reporting requirements.

State of the art report on "Survey of Literature on Crevice Corrosion"

A literature review is presented on the techniques which are used to obtain data on crevice corrosion, both regarding the mechanism and the application of alloys. In particular the electrochemically oriented techniques are covered. After a short introduction in chapter 1, the theoretical aspects of crevice corrosion mechanisms and models are presented in chapter 2, followed in chapter 3 by the techniques used in crevice corrosion testing. In chapter 4 the bulk of the literature is discussed regarding modelling, test methods and obtained experimental results, and experience of service application of specific alloy types. Finally attention is given to protective methods in chapter 5 whereas crevice corrosion monitoring is discussed in chapter 6.

Crevice formation technique

When testing resistance to crevice corrosion it is necessary to create a crevice between the steel surface and a crevice forming material, usually a polymer. The geometry of the crevice formed has to be accurate and reproducible if reproducible results shall be obtained. The severity of the crevice and deeper crevices are the most severe for initiating crevice corrosion in an environment containing chloride. This result describes a technique to form a defined crevice on a stainless steel test specimen. It is recommended to use a spring loaded crevice assembly and use PVDF as the crevice forming material.