Simplification of maintenance for bolts and fasteners

Paul Boughton

There are manifold reasons to avoid corrosion in maritime applications above-sea or sub-sea. Suitable standards to protect parts like boltings and fasteners for many years from corrosion are the cathodic corrosion protection or the use of cost intensive duplex and super duplex steels.

Cheaper protective coating systems have not, until now, delivered sufficiently on the requirements that sub-sea applications premise.

Zinc-based coating systems for example zinc+PTFE, zinc-flake, zink-nickel or hot dip galvanised zinc tend to create voluminous corrosion products once the suface has cracked, which can already happen while assembling the parts.
 
Therefore exchanging parts like stud bolts or other fasteners while performing maintenance can already be troublesome above-sea and even more sub-sea. The corrosion products prevent the parts form being dismantled easily and very often the parts need to be cut.

Electrodeposited Aluminium can prevent these issues. Several independent tests and over 15 years of field experience have shown that electrodeposited aluminium with a PTFE top coat can deliver on the requirements for above-sea, splash water zone and sub-sea applications.
Salt Spray Tests according to DIN EN ISO 9227:2006 delivered results of over 10.000 hours without any signs of corrosion.  

Stud bolts coated with electrodeposited aluminium and a top layer of PTFE-based lacquer have also been assembled on offshore   rigs in the North Sea for over 15 years without the need of exchanging them. The parts are still in use.

What differentiates this surface technology from other available coating systems is its very dense, fine crystalline and ductile structure. The ductility prevents the coating from flaking or cracking even after several assemblies.

Independent tests carried out by the IFUM at the University of Hanover have shown that the surface does not break before the substrate does. The PTFE layer delivers a coefficient of friction of 0.07 to 0.11 to ensure easy assemblies. Substrates that can be coated are casted steel, mild steel, stainless steel and high strength steel. High strength steel can be coated without the loss of the desired hardness and without the risk of diffusing hydrogen into the metal. Tempering after the plating process is not necessary.

The prevention of hydrogen embrittlement results from the electroplating process, because it is conducted in aprotic, organic solvents, in which the aluminium is dissolved as an aluminium-organic complex. To protect these electroplating solutions from humidity and oxygen, the electroplating of aluminium is performed under an inert gas atmosphere of nitrogen or argon. For anodic material aluminium is used, which has a purity of >99.7 per cent. During electroplating of aluminium a purification (electrolytic refining) takes place, which results in an aluminium coating with a purity of 99.99 per cent Al per centage by mass. The high level of purity of the aluminium forms the basis for the corrosion protection. The corrosion protection of the aluminium coating can be increased by common procedures of aluminium after-treatment, e.g. conversion coatings and PTFE-based lacquer.

The corrosion behaviour in chlorine-containing media is based on the fact that pure aluminium has a low self corrosion because the aluminium is passivated by a thin oxide layer due to air oxygen. The oxide layer of the pure-aluminium coating is stable in the pH range from 4.0 to 8.5. Damaging the coating does not result in voluminous corrosion products. In addition the electrodeposited aluminium coating cathodically protects the substrate when the coating is damaged. Electrodeposited Aluminium with a top layer of PTFE based lacquer can deliver a lifetime corrosion protection for boltings and fasteners used in sub-sea or above-sea applications allowing for easy handling in maintenance due to its low coefficient of friction and thin corrosion products when the coating system is damaged.

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