Technical Presentations at the October 2001 Meeting

'Why or is there a gap between laboratory test data and field experiences using high alloyed SS for seawater handling - Crevice corrosion and CSCC susceptibility,
Trond Rogne (SINTEF).

There has been and will be in the future a lot of discussion with respect to reliability of test methods. This is largely because there exist numerous papers where CCT and CSCCT show scatter over a wide range. For instance, for one 6 Mo material we can provide CCT data ranging from 20-25 to 55-65°C depending on test method. Over the past few years the gap has been closing between the different sets of published data as we have become more aware of the effects of differences in testing.

High alloyed Stainless steels (6Mo, Superduplex, duplex) were introduced as candidate materials for seawater applications in the early 80'ies, and the experience with these materials varies from excellent to poor. Severe corrosion/cracking has been experienced with failures caused by bad material quality or service conditions which are uncontrolled. As a response NORSOK has placed a limitation with respect to use for these materials: Max. 15°C if crevices are present, 30°C without crevices, and regarding CSCC, max operating temperature should be 100-110°C.

Factors of influence on the safe operation limit are: threaded connections, gasket type in flange connection, aging effects, high temperature fluctuations, temporary high chlorination levels, unsatisfactory weld procedures and heat treatment procedures of flanges, and natural scatter in material properties.

Test methods as basis for material selection: most results from lab.tests have shown that the high alloyed SS can be used at least up to 25-30 °C although Failures far below 30 °C have been reported. Stock materials, site welds and special products should to a higher extent be used for testing and R&D, rather than materials specifically produced for the test programme. Also the expected scatter in material quality should be evaluated.

How should testing be done to predict service performance? What qualification criteria should be used? Will specified material quality be documented through standard testing? Pitting test often required; What test temperature; Consequences of the presence and amount of IMP's (inter metallic precipitates); Few replicates are required and statistic treatment of data are impossible; 50 °C with pickling, 45 °C without pickling? A test for qualifying crevice corrosion should be required and at what temperature?

Test methods are often not based on giving values for specific application but for ranking. Therefore: Test methods applied are mostly good for ranking, but not always; For application not so good often due to -Users missing knowledge of testing and consequences of differences between test conditions and service -Design and service conditions differs with respect to temperature and chlorine as most important for crevice corrosion and salt concentration/solution composition as the most important for CSCC; Test methods are simplified; Bulk environments are simulated, NOT what can be obtained under worst case scenarios; Simple test assembles are applied most often not reflecting critical conditions with respect to geometry affecting the results; Direct link between lab. test data and service often misses; Due to simplification lab.testing often give too high values; Not evaluated for purpose. Also data are often taken as "exact values instead of guiding" due to the way they are presented - It's our job to improve on this aspect avoid "fooling" the less trained users!!!

References: NACE CORROSION '98, paper No. 696-"Weld o.lays of Ni alloys"
Eurocorr '99-"Chloride Stress Corrosion Cracking"
Stainless Steel '99-"Crevice corrosion and test methods"

Hydrogen embrittlement Stress Corrosion Cracking of Superduplex Stainless Steel Under Cathodic Protection
Paul Woollin (TWI).

Two superduplex stainless steel hubs on a subsea manifold subject to cathodic protection failed as a result of hydrogen embrittlement stress corrosion cracking. A series of tests was performed to establish the threshold condition for cracking. The tests included: (i) constant-load smooth bend tests, (ii) constant-deflection smooth bend tests, (iii) constant-load pre-cracked bend tests, (iv) interrupted slow strain rate tensile tests, (v) constant-load tensile tests, (vi) approximately constant-strain tensile tests and (vii) full-scale hub tests.

The testing identified a very marked difference in material response under load-control and displacement-control. Under constant-load conditions, tensile testing indicated a threshold stress for crack initiation and propagation in the hub material in 50 days of 545MPa, equivalent to an initial strain of 0.5%. Strain continued to develop over the test duration, due to low temperature creep, to 0.9% after 50 days. Full-scale hub tests confirmed that this threshold level was appropriate to the hubs and that residual stress in the hubs contributed to cracking. In displacement-controlled bend tests, with deflection prior to exposure, threshold strains of 2.1% and 8% were identified for crack initiation and propagation respectively.

Comparison with previously published work and powder metallurgy pipe indicated that the hub material was particularly sensitive to hydrogen embrittlement stress corrosion as a consequence of its microstructure, which had coarse aligned grains and nitrides/carbonitrides. Ferrite volume fraction and hardness were apparently of secondary importance.

25%Cr Superduplex Stainless Steel - 35 years old and still going strong(er)
Clive Tuck (Langley Alloys, Meighs Ltd.).

25%Cr duplex stainless steels, which have become known as 'superduplex' stainless steels, developed from the ACI casting alloy CD-4MCu of the 1950's. The first commercial wrought superduplex was FERRALIUM 255 (UNS S32550). This was developed through the addition of nitrogen to a composition similar to CD-4MCu and was patented by Langley Alloys in 1965.

Since that time, superduplex steels have continued to develop through market-led and technology-assisted methodologies. This has resulted in the latest grade of FERRALIUM 255 - SD50 which represents the highest specified superduplex with minimum 0.2% Proof Stress of 570 N/mm.2 and is the fifth upgrade of the FERRALIUM 255 alloy. The three main superduplex stainless steels currently in use are UNS S32550, UNS S32760 (ZERON 100) and UNS S32750 (SAF 2507).

Due to the complex nature of the superduplex alloy system and its relatively low heat conduction characteristics, sigma phase is impossible to avoid during manufacture of superduplex in large sections. UNS S32550 has historically been found to be the most forgiving of the three main superduplexes in this respect.

Electrochemical studies have shown that, in chloride solutions, pit initiation occurs in the ferrite phase, although this phase generally has a higher PREN than the austenite phase. Pit growth is inhibited by impingement on adjacent austenite phase, as shown by potentiostatically produced metastable pitting current transients.

Superduplex with no copper content shows a reduction in active current if copper chloride is added to 1M HC1, producing similar electrochemical behaviour to superduplexes containing copper. Thus, a possible mechanism for the action of copper in superduplexes such as FERRALIUM 255 - SD50 is the cathodic formation of a film of insoluble copper chloride which prevents the formation of active pits.

Post Fabrication Cleaning: Some "Why's and Wherefore's"
Chris Baxter, (Avesta Polarit Ltd).

Surface finish, of the parent material and of the weld zone, has considerable implications on the technical and commercial performance of stainless steel. The importance of specifying and achieving an appropriate surface finish cannot be over emphasised. The surface of the steel can be contaminated during fabrication by, e.g., metallic pick up, scratches and inevitable weld zone oxide. Surface condition and contamination can significantly influence the technical and commercial performance of the stainless steel. Many standards and specifications, at industry, national and international levels, require this contamination and weld oxide to be cleaned off. However, little or no guidance is given on how or when this oxide should be removed.

This paper gives results and data showing the implications of surface finish of the parent material and post weld cleaning on technical and commercial performance. Bright Annealed, SuperBrush(R) and 2B finishes are considered. The technique of weld oxide removal has a significant affect on the performance of the joint. The implications of mechanical and chemical post weld cleaning techniques are shown for 300 series stainless steels.

Practical post fabrication cleaning procedures are generally inadequately specified, even though standards do exist. Consideration is given to specifications used in production.