EXPLORATION • DRILLING • FIELD SERVICES


and the effect of the process on non- destructive testing methods such as eddy current testing and MPI. Although several link conditions were


evaluated, for the purpose of this article a standard powder coated link will be compared with a zinc thermal diffused and powder coated link. Fig. 1(a) to (c) demonstrate the test rig and compares the visual appearance of the in-situ testing of the two link types after 18 months exposure in the splash zone. It is clear from Fig. 1(b) that the zinc thermal diffused and powder coated link was still in good condition after the exposure period. Te traceability markings were also clearly visible. Te standard link on the other hand shown in Fig. 1(c) exhibited excessive uniform corrosion with the powder coating peeling off and the traceability markings being illegible. Salt spray tests were carried out in


accordance with the requirements of ASTM B117-02:2002 for a duration of 600 hours with inspection intervals of 200 hours. Four link conditions were evaluated during the salt spray test, namely: natural finish; shot blast and powder coated; zinc thermal diffused; and zinc thermal diffused and powder coated. For this case study the standard shot blast and powder coated link was compared with the zinc thermal diffused and powder coated link. After 600 hours of salt spray testing the standard powder coated link had a rust rating of Ri4 compared with a rating of Ri0 for the zinc thermal diffused and powder coated link. Tis implies that the zinc thermal diffused and powder coated link showed no significant evidence of rust after the salt spray test period of 600 hours. Te results of these corrosion resistance


tests have been supported by analysis and a comparison of the corrosion effects on ‘treated’ (meaning zinc thermal diffused and powder coated) and ‘untreated’ (meaning powder coated or painted) chain sling products examined after 20 months of service life in a marine environment, as evidenced by Fig. 2(a) and (b). An examination of zinc thermal diffused chain sling products after 30 months of service life demonstrate the improved resilience of treated products as evidenced in Fig. 3(a) and (b). Zinc thermal diffused test plates were submitted to test the adhesiveness of the zinc thermal diffused layers. Tests were conducted in accordance with the


requirements of ASTM D522: 2001 and the results indicated that the zinc thermal diffused layers exhibited excellent adhesive properties. Tis can be ascribed to the fact that the diffused layers are properly bonded with the metallic substrate.


A 50mm master link and 10mm long link chain both in the zinc thermal diffused condition were tested in accordance with IEC/SANS 60079 -15: 2010 for its ability to resist the build- up of static loads. For a product to be declared spark


free a value of less than 1 Giga Ω must be registered during the test. Values of respectively 11.9 Ω and 0.3 Ω were registered for the chain and the master link, which means that both these products can be considered as spark-free products according to IEC/SANS 60079- 15, 2010


Both eddy current and MPI methods


were evaluated on a zinc thermal diffused product and the results indicated that both methods were effective in locating defects on a zinc thermal diffused product.


THE BIGGER PICTURE Tis research together with the case study test results indicate that the sherardising process is well suited for Grade 8 steel alloy safety critical lifting equipment and brings with it several advantages, including: excellent corrosion resistance; no risk of embrittlement in the manufacturing process; and reduced risk of HE in service. Te sherardising process is also environmentally friendly and is a sustainable option due to it extending the service life of the product. Further advantages include: the excellent adhesive properties of the zinc alloy layer; the fact that the product has anti spark properties; and that the process does not interfere with the mechanical properties of the components. Finally, from an operational perspective, the advantage is that users can accurately conduct non-destructive testing at inspection intervals. Further research is needed to identify,


and more accurately quantify, the economic and lifecycle benefits of using sherardising for corrosion protection. Such research is now being conducted by McKinnon Chain and William Hackett Lifting Products to determine the estimated life cycle of zinc thermal diffused products both in the subsea and in splash zone conditions. Samples will


Fig.3 (b)


Fig.3 (a) and (b) Zinc thermal diffused and painted chain sling after 30 months service


Fig.3 (a)


be removed periodically (monthly) where the weight loss and thickness of the coating will be measured. Te project will take place at the East Coast of South Africa and will have a duration of approximately six months. Te results of this research will be published when available and subsequently used to inform a cost benefit analysis of treated versus untreated Grade 8 steel alloy lifting chain and link products.


Rod Bell is technical director at William Hackett Lifting Products www.williamhackett.co.uk


www.engineerlive.com 17


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