When selecting a suitable vessel-protection system, one needs to not only look at the cost of a unit, but at total expenditure. This can include coverage rate, length of downtime and costs of application based on its simplicity or difficulty, among others. The list goes on and, ultimately, a system that is cheaper to procure may turn out to be more expensive once total expenses are tallied.

Here is a look at more aggressive environments of elevated-temperature immersion service with relatively corrosive acids or alkalis and some degree of erosion or abrasion. These processes can be common in piping and process vessels in the oil and gas sectors, as well as petrochemical, chemical and mineral-processing industries. Chemicals such as sulfuric and hydrochloric acids, in combination with seawater and erosive slurries, can quickly eat away at the base metal—typically carbon steel.

The industry has moved on from high corrosion allowances of the base metal to applying corrosion protection, which is able to extend the lifetime of the asset to an average between 20 and 30 years.

Material selection for protecting the base metal can include a variety of options—exotic alloys, metallic cladding, epoxy, and rubber linings, among others. Exotic alloys and metallic cladding understandably bear very high capex, whereas the rubber-lining option is relatively cheap. But, due to its permeability and swelling potential, it is not the most reliable corrosion-protection solution against harsh chemicals.

Alternative options seem to sit in between, and have advantages as well as risks that may be associated with their use. Here is a description, giving some recommendation when each would be more suitable.

Glass linings

Glass-lined steel provides superior corrosion resistance to acids, alkalis, water and other chemical solutions—with the exception of hydrofluoric acid and hot concentrated phosphoric acid. As a result of this chemical resistance, glass linings can serve for many years in environments that would quickly render most metal vessels unserviceable.

At higher temperatures, glass is not as effective against alkalis, where an increase by 18 degrees Fahrenheit (F) doubles the rate of attack on glass. Glass performs well in a variety of operating conditions, offering excellent resistance to corrosion. Its anti-adhesive properties make it very suitable for use in the chemical and pharmaceutical industries.

Of course, because a glass lining protects in extremely aggressive environments, its costs are directly proportional. In addition, it is very susceptible to impact damage and the repairs can be very costly. For milder service conditions, a glass-flake technology can be considered.

Glass-flake coatings

Glass flakes have been used to improve barrier properties and reinforcement in anti-corrosive coatings since the early 1970s. Nowadays, glass-flake-based coatings are used in a variety of industrial sectors due to their good chemical and erosion resistance.

There is relatively poor understanding of how the glass bonds within the various resin matrices. And, although glass flake is impervious to moisture vapor and gas diffusion, it does not present a continuous barrier in a resin matrix. The resin carrier, therefore, plays a very important role—i.e., glass flake cannot make a poor resin film into an excellent coating, although it may substantially improve it.

Vinyl ester is one of the more common resins used with glass flake, which offers benefits in terms of cost saving, but also has several drawbacks. The polymerization process involved in curing a glass-flake system leads to shrinkage, causing the bond line to be permanently stressed. Adhesion is also found to be inferior to that of an epoxy-based system.

The system can also be brittle and easily damaged during inspection or maintenance. The cure mechanism, inhibited by atmospheric oxygen, can lead to significant coating voids that will lead to failure, particularly in decompression situations, which was confirmed by a test sponsored by a global group of energy and petrochemical companies.

Vinyl ester glass-flake systems can therefore be quite suitable for pipeline protection or storage tanks, but not ideal for use in pressurized equipment.

Organic epoxy coatings

Epoxy coatings such as ceramic-filled, modified epoxy novolac (phenol formaldehyde resin) or high-molecular-weight polymer composites have been on the market for many years and have been continuously modified through the use of new raw materials to improve temperature resistance, abrasion resistance, adhesion, sprayability for ease of application, and other features.

Ceramic-filled epoxies are very widely used for erosion-corrosion protection. The first application on a process vessel was carried out in 1987 when a separator was protected at a North Sea platform in the U.K. Their limitations are temperature resistance and sprayability. Both were later addressed by the introduction of modified epoxy novolac, which continuously resists immersion temperatures up to 320 F, and high-molecular-weight polymers, which offer superior erosion resistance while being spray-friendly.

There are some risks associated with the use of epoxy coatings, mainly applicator error and incorrect coating specification. Both can be addressed with appropriate training and guidance by the material manufacturer.

Where epoxy coatings are limited in terms of temperature and chemical resistance, non-stick polytetrafluoroethylene (PTFE) coatings can be used.

PTFE pros, cons

PTFE coatings are very widely used in situations in which superior corrosion resistance is required. Fluoropolymers are the materials of choice for the process industry, serving as linings for vessels, piping, pumps, valves, columns, column internals, hoses, expansion joints, seals and gaskets. They provide durable, low-maintenance alternatives to exotic metal alloys, offering thermal stability for use at high temperatures and, because they do not react with the process liquids, they prevent contamination.

PTFE is virtually inert electrochemically, biochemically, enzymatically and chemically. Important useful properties—that is, not more than 15% loss of chemical resistance—are retained at up to 392 F, giving PTFE the highest retention of its chemical properties of any known plastic-like material.

Unfortunately, PTFE comes with several unhelpful properties when used for moving corrosive materials around, and it is necessary to understand these to manage them. Because of a lack of intermolecular forces, the material is soft and easily abraded. Thus, erosion is a potential concern, as is the property of creep or cold flow under load. PTFE is also difficult to repair if damaged, as this cannot be done in situ.

Here is an example of this in action.

A polyolefins petrochemical plant based in Ferrara, Italy, needed a new coating system for its reactor, operating between 158 F and 176 F and processing salt, caustic and titanium tetrachloride. The original, hot-applied PTFE lining required maintenance due to localized disbondment caused by minor abrasion of the titanium compound. As a result, the plant faced downtime between two and four weeks because in situ repair was not possible.

The maintenance manager wanted to keep the downtime to a minimum and decided to replace the PTFE with a 100% solids-modified epoxy novolac system, Belzona 1593, that is designed for elevated temperature immersion of up to 320 F. Belzona 1593 was hand-applied onto the reactor, facilitating its return to service within four days.

An added benefit considered during material selection was the servicing of the lining. As this system is applied at ambient temperatures and adheres well to the metal and itself, it can be repaired in situ when necessary. A similar epoxy system, for instance, has now remained in service on a test separator at a North Sea platform for 20 years, being patch repaired once every decade during regular inspections.

The reactor was opened for inspection some six months later. If the lining were to start failing due to poor chemical resistance, this first inspection would have revealed visible signs of degradation. The lining was found to be fully intact.

Moreover, the plant was able to save time by steam-cleaning the reactor, which was not previously possible with the PTFE lining. Now, options to replace the lining on other reactors, as well as protecting new reactors with the same 100% solids-modified epoxy novolac system, are being considered.

Savings resulting in reduced downtime—along with simplified cleaning and maintenance protocols—significantly outweighed the initial cost of material in this case.

Introduced in 2014, Belzona 1593 is the latest addition to the range of Belzona lining materials designed for elevated temperature immersion. Incorporating rubbery domains into the polymer matrix of this modified epoxy novolac material allows the lining to display a greater flexibility and creep resistance, improving in-service performance.

Questions to answer

Selecting a lining for corrosion protection depends on the answers to several questions. A simplified approach that only considers a few lining features is not sufficient when lasting protection is required. For instance, when specifying protection for a pressure vessel, some of the questions to answer would be as follows:

  • What are the vessel’s design and operating temperatures and pressures;
  • How is the protection system applied? Is there a need to control the environment, prepare the surface, post cure, etc.;
  • How will the system protect the weak areas, such as small-bore nozzles, or would additional protection be required;
  • Can the material manufacturer provide sufficient in-house and independent testing data supported with case studies and other examples of in-service performance;
  • How will the lining respond to in-service changes such as substrate flexing or rapid decompression; and
  • Can the lining be repaired in situ? Can it be cleaned, steamed out and walked on during inspection? In this case, if localized repair is not possible, a periodic, total recoat needs to be factored into the opex budget.

These are some of the issues to consider. Corrosion engineers will have their own lists and guides to follow as well as national and international standards that address corrosion protection. As technological advancements continue, the process for selecting the most appropriate corrosion protection for a given asset is likely to become increasingly complex, and the industry will rely on material manufacturers to be efficient and transparent in communicating their advantages and limitations.

Marina Silva is on the staff of Belzona Poylmerics Ltd., based in Harrogate, England.

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