Confronting corrosion

Lifting operations in a marine environment pose several challenges of which corrosion and the potential embrittlement of steel chain-based components are some of the most important ones from a material integrity perspective.

Corrosion reactions are not only damaging to the surface of the components which can lead to stress raisers and a reduced service life, according to Burgess B (2021), they can also promote hydrogen embrittlement (HE), especially on components that operate in offshore or other corrosive environments.

The manufacturing processes of steel chain, lifting product technology and material properties have developed over many years. The use of Grade 8 steel alloy chain and components is currently acknowledged as the preferred option when it comes to lifting applications in a marine environment. This grade of steel alloy not only complies with the strength/design requirements for safe lifting but also assists in minimising the risk of embrittlement type failures such as HE.

Ancillary to the chain technology has been a recognition of the need for and benefits arising from corrosion protection of untreated self-colour metal. 

Corrosion protection is essentially about maintaining and increasing the functional life expectancy of the equipment, improving economic efficiency, and environmental impacts such as minimising waste, avoiding pollution and sustainability.

Addressing these factors has resulted in the evolution of various corrosion protection methods some of which are categorised in Fig. 1.

The primary considerations for stakeholders are:

• Manufacturing challenges – the corrosion protection method selected must be achievable without detriment to the product’s base material and mechanical attributes, and it must be environmentally sustainable.

• Products must be fit for purpose with a material specification and material properties that complies with the required lifting standards, durability and resilience requirements.

• Lifting equipment markings should be clearly identifiable to facilitate product recognition to satisfy the lifting standards requirement of product traceability.

• Testing requirements: The protection method must be inspection friendly allowing non-destructive test methods such as eddy current and magnetic particle testing to be successfully applied.

• Economic – initial capital expenditure and product life cycle cost

Establishing the mechanical properties of lifting equipment is a relatively straightforward engineering design matter.  However, creating economically sustainable products that satisfy end-user aspirations on product life cycles and minimise environmental impacts requires an understanding of how corrosion protection can be used to good effect.

Historically products such as steel alloy lifting chain and components such as master links and quad assemblies have been manufactured with various types of corrosion protection including paint, powder coating, electro plating and hot dipped galvanising.  However, once placed into service these coatings are prone to breakdown and erosion due to mechanical stressors. Recent research by McKinnon Chain and William Hackett Lifting Products has considered the suitability of sherardising as a premier corrosion protection method – especially for products used in harsh applications and corrosive environments.

Sherardising is a process recognised to be environmentally friendly with extremely low levels of waste and a zero risk of hydrogen embrittlement. The hard abrasion resistant coating doesn’t interfere with the metallurgical properties of the base material, and it has spark resistant properties. A further benefit from a testing perspective is that the coating doesn’t interfere with non- destructive testing methods such as eddy current and magnetic particle testing (MPI) 

So, what is sherardising? The process is described in Fig. 2.

To inform and support decision-making in relation to the design and development of various Grade 8 lifting products, several tests were commissioned which focussed on (1) corrosion resistance; (2) adhesive properties; (3) anti –spark properties, and (4) 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.

Figs 3(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. 3(b) that the zinc thermal diffused and powder coated link was still in good condition after the exposure period. The traceability markings were also clearly visible. The standard link on the other hand shown in Fig. 3(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 to a rating of Ri0 for the zinc thermal diffused and powder coated link. This 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.

The 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 ‘untreated chain’ (meaning powder coated or painted), sling products examined after 20 months of service life in a marine environment as evidenced by Figs 4(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 Figs 5(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. This 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.

This 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

• The sherardising process is environmentally friendly

• Extend the service life of the product

• Excellent adhesive properties of the zinc alloy layer

• The process does not interfere with the mechanical properties of the components

• Enables one to accurately carry out non-destructive testing at inspection intervals

• The product has anti spark properties

• No reduction in legibility of product identification markings

Further research is needed to identify, and more accurately quantify, the economic and life cycle 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 sub-sea – as well as splash zone conditions. Samples will be removed periodically (monthly) where the weight loss and thickness of the coating will be measured. The project will take place at the East Coast of South Africa and will have a duration of approximately six months. The 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