High strength bolting is used to fasten together the sections of large, cast swivels for the oil and gas industry.  Traditionally 17-4PH stainless steel has been used when carbon and low alloy steels are not suitable.  A new project required a maximum operating temperature of 60°C in a tropical marine atmosphere.  A review of the existing data and service experience showed that there was no information above 40°C.  Testing was, therefore, required and a test method was devised to generate data in a relatively short space of time.  This utilized a salt spray cabinet at 60°C with a cycle of 30minutes spray and 30 minutes drying for 1,000 hours.  Five full size M33 stud bolts were tested at 75% of the SMYS (725MPa).  Examination at the end of the tests showed no significant corrosion, including crevice corrosion and stress corrosion cracking.  Mechanical testing of the bolts showed no loss of strength and a 100% ductile failure.  The swivels with the 17-4PH bolts have been accepted and are now in service with no problems to date.

6.1  ‘Experimental Impressed Current Anode Design’, Barry Torrance (Aish Technologies)

New developments in the accuracy of corrosion-related signature prediction and corrosion monitoring:

Signature and corrosion prediction has in the past needed to make assumptions about the direction of current flow around a vessel. For present purposes these assumptions may be ‘good enough’, but as attention focuses on littoral operation in close proximity to mines, and when cost reduction through Condition Based Maintenance becomes a priority, a more accurate prediction is required.

This presentation describes experimental work on a novel type of anode assembly which will contribute to such enhancements.

6.2  ‘Marine Microbiological Corrosion – are we getting to the bottom of it?’, Bob Edyvean (Sheffield University)

Résumé:  Microbiologically Induced Corrosion (MIC) can be due to a number of different organisms – seaweeds (open water) or various bacteria.  Bacteria are the most common and they mostly act in consortia, creating small electrochemical micro-reaction plants.  They are more virulent on surfaces, needing up to five times the quantity of biocide to remove them than if they were free ranging.

Corrosion reactions are accelerated, and this appears in a highly localised form (pits within pits).  Hydrogen generation is also stimulated and hydrogen embrittlement or internal hydrogen damage can occur.  Samples for examination should never be cleaned.

Mitigation against MIC takes the form of system cleanliness maintenance, the application of coatings or cladding, and use of biocide of the correct concentration.  Cathodic protection can stimulate anaerobic bacteria and the calcareous deposits which form about bacteria.  Paints can also attract bacteria – anti-fouling paints should be specified which are applied on top of standard paint systems.  CT

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