Technical Presentations at the April 2017 Meeting

1.1 'High Alloy Stainless Steels for Seawater Heat Exchangers', Nicolas Larché, Institut Corrosion, France

Heat exchange involving seawater (natural or chlorinated) is a demanding application in terms of material corrosion resistance and titanium alloys are commonly used for this purpose, especially at temperatures exceeding 40°C. However, titanium can be expensive and difficult to supply, and looking into alternative alloys such as high-alloyed stainless steels becomes of interest for these applications. Due to the potential susceptibility of stainless steels to localized corrosion in chloride/chlorine containing media, the actual corrosion risk of the candidate materials must be carefully addressed and corrosion resistance will strongly depends on the exact service conditions (e. g. temperature, chlorination, oxygen content, etc.), on the metallurgy (e. g. cast or wrought alloys) and on the geometrical configuration of the areas in contact with seawater, as well as surface contamination.

From this background, several fit-for-purpose studies were conducted to evaluate the corrosion resistance of selected high-alloyed stainless steels for tube heat exchanger applications, including hyperduplex stainless steel UNS S32707 and superaustenitic UNS S31266. Results were compared to the more commonly used duplex UNS S32205, superduplex UNS S32750 and superaustenitic UNS S31254… The corrosion resistance was evaluated using full scale tube heat exchangers, involving chlorinated seawater flow loops. In parallel, laboratory exposures have been performed to assess the limits of use of the selected stainless steel grades regarding temperature and residual chlorine content.

1.2   'Stainless Steels and Chlorides - A Good Idea or Not?', Robert Conder, Engineering Materials Solutions Ltd

The presentation looks at the issues with austenitic stainless steels and 22%Cr duplex alloys in the presence of chloride ions, high temperatures and oxygen of various concentrations and will aim to explore what is known about the performance and limits of these materials in various environmental combinations

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3.1    ‘Non-Destructive Testing in a Marine Environment’, Patricia Conder & Graham Marshall, Sonomatic

The marine environment poses many challenges and some benefits for Non-Destructive Testing, to identify and size degradation mechanisms, such as fatigue cracking and corrosion. This talk discusses some of these challenges and the different approaches to meet and overcome these challenges on an industry wide basis. Areas which will be covered include:

• Physical deployment of NDT techniques below the waterline and the challenges posed by water depth.

• Coatings including marine growth- to remove or not?

• Measuring corrosion from the corroded side and the impact of rough surfaces.

3.2  ‘Assessment of Effects of Cavities and Narrow Channels on CP Design in the Marine Environment’, Tim Froome, BEASY

CP system design work in the marine environment makes use of structure surface area, required current density, and anode resistance formulae to select the number and mass of anodes required to protect a structure.

The formulae cannot take account of restrictions to current flow caused by close proximity of structural surfaces to each other, and consequently CP systems which have been designed in the recommended way may sometimes not protect parts of a structure.

In some situations it may be judged that cathodic protection is not required on some surfaces, but recommended practise is that current drain to such surfaces needs to be taken into account in the design.

This presentation first investigates the general effects caused when cavities and narrow channels/annuli are present, and attempts to identify patterns of behaviour. A series of computer-based parameter studies is performed in which, for example, separation between two structural surfaces is varied, and the extent of the protected area is measured.

Secondly this presentation investigates effects of geometry on current drain to surfaces which do not need to be protected. Again computer-based studies are used, and for example the size of holes is varied.

3.3   ‘Concerns Over Utilizing Aluminium Anodes in Sealed Environments’, Alex Delwiche, Deepwater EU Ltd

A CP Trial was carried out offshore to determine the requirements and monitor the hydrogen levels inside the submerged section of a Wind Turbine. The results were presented and explained that these were unexpected. Water tests concluded that the aluminium anodes were contaminating the water and turning it acidic. The presentation discussed several theories as to why this could occur; namely by the aluminium chloride by-product of the anode dissociating in the water, and presented a comparison between the theoretical pH calculations, with those results found in the field and also those carried out in a laboratory. The theoretical and practical results were found to be reasonably similar however the presentation postulated that the results in the field could have been affected by the as-found reduction of carbonates namely bi-carbonate as a result of the corrosion process before the CP trial was undertaken.

A remote monitoring system was key to understanding the issue which has never been identified in the past, for example inside of water ballast tanks and concrete gravity leg structures. The monitoring system comprised of several reference electrodes, a current density plate, the anodes current output and hydrogen sensors. Notably the Hydrogen sensors didn't sniff hydrogen gas levels of any significance

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