Technical Presentations at the October 2014 Meeting

1.1   Improving HISCC Resistance of ZERON 100 Super Duplex Stainless Steels’, Glenn Byrne, Rolled Alloys

The presentation reviewed the history of HISCC failures of duplex and super duplex stainless steels when deployed subsea and subject to CP at potentials around minus 1V. The test methods applied to investigate the problem and subsequently used to develop current design codes that deal with HISCC were reviewed. Data from these investigations was compared with other testing using the same cast and batch of material but where controlled shot peening had been used to induce compressive residual stresses in the surface of the test samples. Patent material and cross welded samples were tested. Peened material showed a 10 to 15% improvement in the threshold stress to initiate HISCC.

The presentation also discussed the advanced forging process ("AFP") recently developed to provide both increased notch toughness at low design temperatures and improved resistance to HISCC in forgings used to make API weld neck and swivel ring flanges for subsea manifolds. Data was presented showing an increase in the threshold stress from 85% SMYS to 97.5% SMYS. This was attributed to the dissolution of detrimental nitride precipitates within ferrite grains in the forgings and transforming these in to advantageous intragranular reformed austenite.

[A pdf version of this presentation has kindly been provided for members, and can be obtained from the Secretariat].

1.2  Corrosion Issues on Ships’, Clive Tuck, William Wistance and Colin Waylen Lloyd’s Register EMEA

Most ships are made up of welded steel plates and current technology of coatings enables corrosion to be effectively prevented during their operation in seawater. This, together with the fact that adequate corrosion protection of the steel ballast tanks is now subject to international legislation means that there are very few issues of corrosion concerning the main structure of ships.

However, corrosion issues still occur from time to time on various other parts of ships and several examples were presented which have been investigated by Lloyd’s Register and the types of corrosion seen were described under various categories.

Examples of galvanic corrosion concerned an aluminium vessel with 6082 alloy stern tube and 2205 duplex stainless steel propeller shaft where the forward seal was not electrically isolated from the hull and other cases of lack of electrical isolation (CuNi/Titanium and Brass/Bronze) in the seawater cooling system of ships.

The problem of ensuring adequate throwing power of ICCP systems along the full extent of hull penetrations was discussed in relation to a corrosion issue seen on an example of a seawater outflow pipe. Penetration depth should be a maximum of four times the penetration pipe diameter to enable effective ICCP operation.

Sometimes design deficiencies come to light through incidences of corrosion. An example concerned a diving vessel, where 316 stainless steel piping was used on the hot water system for the divers. The normal temperature of operation was 30°C but the temperature of the seawater went up to 45°C on a fairly regular basis which resulted in crevice corrosion after a few month’s operation.

Some examples were presented of poor maintenance causing corrosion, such as cases of lack of dry scrubbing of exhaust gases above cargo tanks containing sour crude oil, lack of adequate greasing of wire ropes and the problems associated with stagnant conditions developing in pipework (particularly copper alloy and stainless steels) due to long times spent in harbours. It was emphasised that the initial design review needs to be robust and that operating and maintenance procedures for ships need to be strictly followed.

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  4.1  Microbiologically Influenced Corrosion in Offshore Environments. Have we seen the limits of microbial action?’, Felipe Leon Morales, Endures (TNO), Netherlands

Microbiologically Influenced Corrosion (MIC) has been known to dramatically increase corrosion rates in many environments where microorganisms are present and active. Traditionally, MIC has been believed to occur exclusively in non-extreme environments (room temperature, atmospheric pressure, neutral pH, etc). Normally extreme conditions are believed to be safe as far as MIC is concerned. Increased industrial activity in zones considered extreme such as deep sea mining or arctic operations is becoming every time more common. Little is known about the potential for MIC in such environments.

From microbial ecology research it is known that microorganisms can be active in some of the most extreme environments on Earth. The question arises about how corrosive can such organisms be for commonly used metals. We have tested MIC activity (by now using carbon steel) at low temperatures and high pressures typical of current industrial applications. At low temperatures (4°C and 10°C) we have found microbial corrosive activity comparable to that at room or physiological temperature, provided microbial cultures are adapted to grow at such conditions. Similarly, under high pressure, we found growth and corrosive activity from an unadapted model sulphate-reducing microorganism (Desulfovibrio indonesiensis) at 100 b hydrostatic pressure.

These findings emphasize the need of taking into account the possibility that MIC might occur even though conditions remain relatively extreme. The main challenge faced is access to such environments for analyses, which at present relay mostly on offsite laboratories. Not many in-situ microbiological technologies are available at present but will be highly required in the near future.

[A pdf version of this presentation has kindly been provided for members, and can be obtained from the Secretariat].

 

4.2  Corrosion Performance of Metals for the Marine Environment - a different perspective?', Carol Powell, Copper Development Association & Nickel Institute.

There are many different alloys available to engineers for structures, systems and components for use in sea water. These include steels, stainless steel and alloys of copper, nickel, aluminium and titanium. Good performance relies on an understanding of their corrosion behaviour and a requirement that they are selected and designed to their strengths and not their weaknesses. Many systems are mixed metals and knowledge of the compatibility of alloys in contact with each other is also crucial.

Different alloy groups have different methods of corroding depending on the sea conditions. Flow, temperature, stagnation, pollution, applied stresses and aeration can all produce a different response. Some alloys can show uniform thinning while others may show localised corrosion which can include corrosion at crevices, pitting, stress corrosion and corrosion of preferential phases.

This presentation provided a brief overview of the world of metals and their response to one of the most corrosive environments there is, giving examples of the types of corrosion which can occur and how to avoid them. The presentation was given to mark the announcement in a reduction in the price of the EFC/NACE Publication 63 “The Corrosion Performance of Metals for the Marine Environment: A basic guide” by Maney Publishing to make it more affordable to engineers new to the marine environment and students. It can now be obtained from http://www.oxbowbooks.com/oxbow/the-corrosion-performance-of-metals-for-the-marine-environment.html

[Copper Development Association, Hemel Hempstead, UK & Nickel Institute, Brussels, Belgium 1210]

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