The impact of galvanic corrosion on critical assets is an enduring problem for offshore oilfields. At best, it increases the workload and repairs associated with planned shutdowns. At worst it can result in rapid or unexpected damage, leading to unplanned shutdowns.
Either way, it is a costly problem which harms efficiency, putting additional pressure on profit margins that are already under threat. However, recent advances in valve technologies to eliminate galvanic corrosion could provide valuable learnings for the wider industry.
Why does galvanic corrosion occur?
The cause of galvanic corrosion (also referred to as ‘dissimilar metal’ or ‘bimetallic’ corrosion) is an electrochemical reaction between two different conductive materials in close proximity. It results in one – the least noble of the two – being preferentially corroded.
Several conditions can result in galvanic corrosion:
- The metals are far apart on the galvanic series, meaning a significant difference in their oxidation potential.
- The metals are in electrical contact, for instance bolted, welded, clamped or even resting against each other.
- The metal junction is bridged by an electrolyte, i.e. a fluid which can conduct electricity. If conductivity is high – as with seawater – galvanic corrosion will affect a larger area of the less noble metal.
To combat this risk, the received wisdom is ‘don’t mix metals’. However, when it comes to the design and development of assets for harsh offshore environments, things are rarely that straightforward.
The graphite problem
With the decline in use of asbestos for offshore components, use of graphite has surged. This mineral’s extreme range of properties and characteristics makes it an ideal material in many ways. It is soft, flexible and easy to cut, but it has high heat resistance and is almost inert. However, it is also the only non-metal that can conduct electricity. Furthermore, its nobility exceeds that of noble metals including stainless steel, bronze and titanium. Yet it is often paired with these materials in valve manufacture, in the form of seals, packings and gaskets.
Figure 1. Engineer inspecting a valve seal.
So, from a galvanic corrosion perspective, graphite has become a thorn in the side of offshore operators. It is the most suitable material for many components, but its use necessitates regular (and costly) repair and replacement of products.
Efforts to overcome this include coating graphite parts in epoxy resin to shield them from seawater. However, in prolonged offshore use, the parts inevitably absorb moisture.
A study conducted by the Naval Surface Warfare Center in the 1990s examined the impact of seawater environments on the galvanic corrosion of graphite epoxy (GR/epoxy) composites coupled to metals. Results showed that:
“…galvanic corrosion of HY80 [a low alloy steel] and NAB [Nickel Aluminium Bronze] will occur when these metals are individually coupled to Gr/epoxy composites with exposed graphite fibers and immersed in quiescent seawater…Galvanic corrosion can also occur between Gr/epoxy composites with no graphite fibers initially exposed to the environment and a metal. This corrosion is believed to be due to moisture absorption through the epoxy outerlayer to the graphite fibers.”
A new way to eliminate galvanic corrosion risk
When a leading oilfield operator commissioned Severn to supply triple offset valves for a seawater service application, the company challenged its design engineers to tackle this issue head-on. Their brief was to isolate or remove graphite components from our patented oblique cone technology (OCT®) triple offset valve, without compromising fire safety credentials. Seawater valves feed water in the event of fire on a platform or vessel, so their ability to operate during and after exposure to fire is a critical safety feature.
Replacing the graphite laminate in the main sealing element posed the most significant challenge. However, the nature of the OCT® design allowed the team to test a series of alternatives and they developed a hybrid seal technology, eliminating the need for graphite. They also removed the graphite gasket, typically found behind the seal on many designs.
To validate design integrity, valve specimens were exposed to a 30-minute burn as part of testing to the latest API and ISO fire test standards.
Figure 2. Valve specimens undergoing a 30-minute burn.
The team also took steps to ensure the valve design could not generate a spark and risk starting a fire itself. They followed the ISO/IEC 80079-36:2018 ATEX standard and ensured that no static charge could build within the valve. This additional element of care renders the new OCT®SW valve design suitable for a wide range of applications, beyond seawater service.
Early adopters of the new technology include an offshore gas compression platform located off Trinidad and Tobago. A total of 17 bi-directional isolation valves have been supplied, meeting stringent requirements for repeatable zero leakage and fire safety, while having no graphite in contact with the line media.
Figure 3. Valves an offshore gas compression platform located off Trinidad and Tobago.
Valves may be a relatively small asset in the context of an offshore platform. But their impact on tier-one equipment as well as overall platform safety and performance makes them critical to productivity and profitability. Eliminating the risk of galvanic corrosion primes them for lengthy, uninterrupted service with minimal maintenance.
Author: Mark Breese, Group Product Development Manager at Severn Glocon
Read the article online at: https://www.oilfieldtechnology.com/offshore-and-subsea/14102019/tackling-galvanic-corrosion-in-offshore-environments/
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