Protecting steel structures in the oil and gas industry from corrosion

Developing new galvanic protective coatings to limit impact upon project coating schedules.

Oil and gas asset owners are increasingly looking for longer performance from their corrosion protection, but also want to reduce their overall capital expenditure budget in all areas.

Protective coatings offer a viable solution to many corrosion challenges found in the oil and gas industry, and currently millions of litres of paints are used to protect these assets globally each year. However, the protective coatings industry has been faced with many challenges in recent years and has had to wrestle with:

  • How to reduce impact on project schedules brought about by late-­stage painting activities
  • How to minimise delays due to painting rework and in-­service failures
  • How to maximise the cost effectiveness of time spent on painting activities

Galvanic coatings contain metal particles that are non-­noble relative to the ferrous substrate. These coatings form a physical barrier and act as a sacrificial anode when the barrier is damaged. Zinc-­containing coatings may be considered galvanic. New galvanic coatings based on organic zinc epoxy resins can significantly improve painting productivity during project construction.

Galvanic coatings often represent part of the largest painting scope on oil and gas capital projects. Therefore, savings made here can offer significant benefits in terms of improved productivity and, subsequently, lower painting costs.

Historically, zinc-­containing paints or protective coatings have been divided into two types:

  • Inorganic zinc coatings based on ethyl silicates
  • Organic zinc coatings based on epoxy resins

In both types, the zinc is present in the form of zinc dust particles. The amount of zinc present plays a key role in determining the degree of corrosion protection provided. The more zinc dust that is present in the final coating film generally leads to reduced corrosion creep when tested in simulated corrosion-­inducing environments (although other factors also play a part). The level of zinc present is prescribed in a number of standards, such as ISO 12944 – 5: 2018 and the Steel Structures Painting Council document SP 20.

The two types of coating also differ in their intended uses. Inorganic coatings can be used to provide corrosion protection across a wide range of temperatures (typically up to 400°C), due to their silicate backbone. Epoxy zinc coatings are limited to providing corrosion protection across a more limited temperature range (usually no more than 120°C) due to their organic backbone.

On the face of it, inorganic zinc-­based paints offer a better option and have historically been found to offer the best level of corrosion protection of the two coatings categories when comparing products with like-­for-­like zinc content. However, inorganic zinc coatings have some significant drawbacks when used for steel protection.

  1. The ideal application conditions for a solvent-­borne inorganic zinc galvanic coating are 20-­25°C (68-­77°F) with a relative humidity (%RH) between 50% and 90%. Painting in conditions outside of this can significantly increase drying time and reduce productivity.
  2. Solvent-­borne inorganic zinc silicates have a tendency to mud-­crack when applied too thickly, leading to reworking costs. Minor mud-­cracking that is tightly adherent may not necessarily be detrimental. However, visible mud-­cracking may be cause for concern. In addition, it may further increase drying times due to the thicker film.
  3. The porous nature of the zinc in an inorganic silicate binder may cause the gassing or pin-­holing of subsequent coats. Techniques to minimise this involve using a mist coat, which may lead to longer application times. Failure to address the issue proactively, however, may lead to subsequent repainting activities and incurred costs.

Given all of the drawbacks, why are inorganic solvent­borne zinc-­based coatings so widely specified and used?

The answer lies in their excellent corrosion protection and their ability to protect ferrous substrates, which has traditionally been considered superior to organic equivalents when comparing products with similar levels of zinc dust.

However, recent advances mean organic zinc epoxy galvanic coatings now deliver comparable corrosion protection, while offering significant construction benefits.

In these improved organic zinc epoxy galvanic coatings, the contribution to the galvanic effect is believed to not only be restricted to the substrate interface. This is because the coatings have an increased ability to release electrons, creating a more efficient anode.