Technical Presentations at the April 2003 Meeting

1.1 Post Fabrication Cleaning: reasons and benefits’, Chris Baxter (AvestaPolarit Ltd)  

Contamination of stainless steel surfaces generally adversely affects the corrosion performance of a unit. This contamination may be from a variety of sources including tooling, ancillary equipment, temporary structures etc. Welding heat tint should also be considered a contaminant in this context.

Pickling and Passivation, two very different processes used in post fabrication cleaning, can significantly improve the corrosion performance of a welded joint. Pickling is often used in combination with mechanical cleaning methods.

It is shown that different technical finishes can be achieved by different post fabrication techniques and that the corrosion performance of the welded joint is directly related to the quality of the technical finish.

It is shown that, for austenitic, duplex and super austenitic grades of stainless steel, the corrosion performance improves as a finer quality mechanical finish is achieved and or pickling is used. It was found that the effect of pickling was much stronger than that of mechanical cleaning.

Clean down time and the amount of water used for clean down is related to the fineness of the surface finish therefore influencing the economic performance of the surface.

It is shown that whilst a variety of passivation products are available and useable for a range of stainless steels, the pickling product should be selected relative to criteria strongly influenced by the grade of stainless steel to be pickled.

Given the clear technical and economic benefits associated with post fabrication cleaning, the emphasis has now changed from justifying why a surface should be cleaned to now justifying why the surface should not be cleaned. Now the premise is that post fabrication cleaning will be completed.

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1.2  ‘Corrosion in our Ports and Harbours’, Roddy James (CorrOcean)

Over the last few years the civil engineering community and the Ports & Harbours industry have become increasing alarmed by the phenomenon which has been termed as Accelerated  Low Water Corrosion (ALWC).  This phenomenon is not new, its just been called another name.  Microbiologically Induced Corrosion (MIC) is common in most industries, with many experts to explain and categorize its occurrence.  It is towards these industries that the civil engineering community needs to look.
MIC is a type of corrosion which is influenced by the presence, and activities of micro-organisms and the products produced in their metabolism.  Sulphate Reducing Bacteria (SRB) are normally associated as the main culprits in the ALWC attack. They are anaerobes that are sustained by organic nutrients.  Circulating (probably in a resting state) in aerated waters, until they find an ideal environment to support their metabolism and multiplication.  Although almost always present at corrosion sites, care needs to by taken because their presence does not always mean they are causing the corrosion.  Key symptoms indicating their involvement in the corrosion process is the black sulphide corrosion products.
Solutions to this type of corrosion tend to point mainly towards cathodic protection as being the optimal technique, however all projects need to be individually evaluated.

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3.1  Experiences with Super Duplex Stainless Steel in Seawater’, R. Francis & G. Byrne (Weir Materials & Foundries).

 Superduplex stainless steel has been used in the cast and wrought forms in seawater systems since 1986.  Since that time the alloy has been very successful in cast form for both pumps and valves.  The wrought product has been used for piping, fittings, flanges, filter vessels, fasteners, mounting brackets etc. under a range of conditions including the North Sea and the Arabian Gulf.  During its use in firewater and seawater cooling systems there has been a small number of failures.  These are mostly associated with welds and examination of these failures coupled with laboratory tests has enabled the safe-use limits for this alloy to be established in terms of chlorination and temperature.  New developments to extend these limits further are described.

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3.2   Nickel alloys in offshore oil, gas and marine applications’, Steve McCoy (Special Metals Wiggin Ltd)

Physical metallurgy of alloys 718, 725, 725HS, 925 for service in aggressive corrosive environments

Nickel-base materials INCONEL® alloy 718, INCONEL® alloy 725, and INCOLOY® alloy 925 are commonly used in oil and natural gas production.  These alloys contain high Cr/Mo contents for aqueous corrosion, and also considerable levels of hardeners like Ti/Nb/Al to form gamma prime and gamma double prime precipitates for strength.  Being heavily alloyed multi-component systems; these materials require special consideration for processing and heat treatments.  Time-Temperature-Transformation (TTT) diagrams can be used as road maps to determine precipitation of various phases under different processing conditions.  This paper shows the effect of intergranular precipitates on mechanical properties and SSR test results in sour oil patch environments.
Processing conditions were designed to produce alloys 718 (UNS N07718), 725/725HS (UNS N07725), and 925 (UNS N09925) with essentially clean grain boundaries, typical of Special Metals standard mill product, and to induce partial coverage of grain boundaries by the precipitates, and full coverage of grain boundaries by the precipitates.  Grain boundary precipitates were found to degrade room temperature impact strength.  Partial coverage of grain boundaries by second phase particles did not adversely affect the properties in slow strain rate (SSR) tests conducted in the oil patch environments.  However, the materials having grain boundaries fully covered with second phase particles had inferior properties in SSR testing.  Matrix and grain boundary second phase particles are preferentially attacked in a corrosive environment due to their different crystal structures, chemical compositions, and depletion of precipitate-forming elements at the precipitate/matrix interface. Continuous grain boundary precipitates are more harmful since these form a continuous network of weak sites. This could explain low SSR ratios for 925-T and 718-M materials, which contained continuous grain boundary precipitates.

The origin of second phase grain boundary precipitates can be related to any of the following processing stages: raw materials, melting/remelting, homogenization, hot working, solution annealing and age hardening.  In the well-processed materials, TTT diagrams can be used as road maps to figure out the presence of various phases under the given time/temperature conditions.  The data showed that the intentional presence of isolated grain boundary precipitates helped to increase the yield strength of alloy 725HS by grain refinement without degrading SSR properties in oil patch environment.  However, intentionally or unintentionally produced continuous grain boundary precipitates in alloys 718 and 925 are shown to degrade SSR ratios.

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