Ben Hayes reveals a new anti-corrosion subsea coating for bolts and fasteners
A new anti-corrosion coating has been developed to specifically address the problem of corrosion and subsequent failure in bolts and fasteners typically used in offshore installations such as wellheads, Christmas tree flanges, structural connections and flanges. These bolts and fasteners can be a problem for maintenance teams as they are often extremely difficult to access and are a major cause of gas or oil leakage in the industry.
A comprehensive range of tests on this new PTFE coating, named Everlube 1174, has been completed with excellent results. The bolts were initially coated using a variety of different pre-treatments followed by the new coating.
The salt spray protection test results were very effective and showed that the new coating will provide: 1,500 hours when applied over a Phosphate pre-treatment; 2,500 hours when applied over Zinc plating; and 5,000 hours when applied over a Cermet basecoat.
Two bolts were tested and after 1,500 hours in the salt spray cabinet as per ASTM B117, the bolts showed near zero corrosion and easy removal of the nut.
This new coating has a high dielectric strength and in testing was also found to act as an excellent insulating shield between dissimilar metals, helping to alleviate the problem of galvanic corrosion.
Determine the appropriate solution
Three key factors are important when determining the correct bolt or fastener system. The first is the grade and type of fastener, in order to match the required operating load performance. Next is evaluation of the installation process; this is equally important to prevent over or under tightening, which could lead to fatigue failure and loosening of the bolt system. Thirdly, identifying which coating system will give the best protection against failure and prolong the life and performance of the bolt or fastener, reducing downtimes and leakage, is crucial.
It is important to understand how the bolts function when determining the correct coating to use. The bolt becomes stretched as the nut is tightened and it is this elastic elongation that will provide the clamping force. The bolt will continue to be stretched until its yield point is reached when it will plastically deform and will no longer be able to return to its original dimensions once the clamping load has been removed. The top limit of the force which can be applied is therefore represented at this plastic yield point.
When choosing the appropriate coating, key considerations are: consistent torque tension properties (K factor); corrosion protection; chemical resistance; UV stability; and colour coding.
Torque values are usually calculated using the following formula:
T (Torque) = K (nut/friction factor) x D (bolt diameter) x P (bolt tension)
A key variable of the K factor is the Coefficient of Friction (CoF) of the coating and Everlube 1174 has been extensively tested on stud bolts and shows excellent torque tension properties. The new coating forms part of the Everlube Products range, which has over time developed a variety of PTFE based coatings with low and consistent CoF values.
Subsea bolting systems also need protection from a wide variety of chemicals, from those present in the sea water to those used in various oil extraction equipment, including hydraulic fluids that are extremely harmful and can typically breakdown low grade coating systems. This new coating has been formulated to provide resistance against all hydraulic fluids currently used in the offshore oil and gas industry.
Bolts are often colour coded and it is essential therefore that the colour remains unchanged, especially as components may be exposed to UV for a time before being commissioned. In the ISO20340 cyclic corrosion test used to represent real-world weathering conditions, the coating showed increased resistance to UV degradation when compared to the current market leader for subsea PTFE coatings.
Increased performance and a significant improvement in pencil hardness has been found in the mechanical properties of the coating, as well as almost double the wear life on the ASTM 2714 Falex block on ring test.
In summary
Everlube 1174 has been developed to prevent the premature failure of bolts and fasteners used in the offshore oil and gas industry and has proven anti-corrosion results in tests, which show between 1,500 and 5,000hrs salt spray protection depending on the pre-treatment used. It will also improve the load bearing capability of the bolt or fastener. The coating has excellent torque tension properties and provides resistance against all current hydraulic fluids used within the industry, as well as improving the load bearing capabilities of the bolt or fastener.
For colour coded bolts and fasteners, its maker says that in tests, this coating showed increased resistance to UV degradation when compared to the current market leader, enabling bolts to retain their true colour. It is available in standard RAL colours.
Everlube 1174 is a low VOC, low friction, thin filmed, resin bonded thermoset PTFE coating that its maker states has exceeded the performance of its key competitors in all relevant performance tests.
For more information at www.engineerlive.com/iog
Ben Hayes is with Everlube Products, a Business Unit of Curtiss-Wright Surface Technologies.
