Choosing corrosion protection for a process vessel or a tank can be a complex task. With so many solutions on the market, what are their advantages and shortcomings
When selecting a suitable protection system, we need to not only look at the cost of a unit, but a 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. In this article we review how selection and maintenance of a corrosion protection can contribute towards the operating expenditure.
We will 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 upstream and downstream sectors as well as petrochemical, chemical and mineral processing industries. Here, chemicals such as sulphuric and hydrochloric acids in combination with sea water 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 of 20-30 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 capital expenditure, whereas the rubber lining option would be on the opposite side of the spectrum. It is relatively cheap, but due to its permeability and swelling potential, it is not the most reliable corrosion protection solution against harsh chemicals.
We will focus on alternative options that seem to sit in between, and highlight their advantages as well as risks potentially associated with their use, giving some recommendation when each would be more suitable.
Glass-lined steel provides superior corrosion resistance to acids, alkalis, water and other chemical solutions (with the exception for hydrofluoric acid and hot concentrated phosphoric acid). As a result of this chemical resistance, glass lining 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 10ºC (18ºF) 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, as a glass lining provides protection to the extremely aggressive environments, its costs are directly proportional. In addition they are 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 containing coatings
Glass flakes have been used to improve both 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 vapour and gas diffusion, it does not present a continuous barrier in a resin matrix. The resin carrier, therefore, plays a very important role, ie, 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 the glass flake, which offers benefits in terms of cost saving, but also has several drawbacks. The polymerisation process involved in the curing process of 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 which 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 pressurised equipment.
Organic epoxy coatings
Epoxy coatings, such as ceramic filled, modified epoxy novolac or high molecular weight polymer composites have been on the market for many years and are continuously “modified” with the use of new raw materials to improve their features, including temperature resistance, abrasion resistance, adhesion and sprayability for the ease of application.
Ceramic filled epoxies are very widely used for erosion-corrosion protection with the first application on a process vessel carried out in 1987, when a separator was protected at a North Sea platform in the UK. Their limitation comes in the form of temperature resistance and sprayability with both of these issues later addressed by the introduction of modified epoxy novolac, which are able to continuously resist immersion temperatures up to 160ºC (320ºF) 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 presented in the form of applicator error and incorrect coating specification. Both can be addressed with appropriate training and guidance provided by the material manufacturer.
Where epoxy coatings are limited in terms of temperature and chemical resistance, PTFE based coatings can be used.
PTFE coatings are very widely used in situations where 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 electrochemically, biochemically, enzymatically and chemically virtually inert. As important, useful properties (that is not more than 15% loss of chemical resistance) are retained at up to 200ºC (392ºF), giving PTFE the highest retention of its chemical properties of any known plastic like material.
Unfortunately PTFE comes with a number of unhelpful properties when used for moving corrosive materials around and it is necessary to understand these in order to manage them. Because of a lack of intermolecular forces, the material is soft and easily abraded. This means that 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 can not be done in situ.
Here, we review an example of this occurrence in action.
A Polyolefins petrochemical plant based in Ferrara, Italy, was looking for a new coating system for their reactor, operating between 70ºC (158ºF) and 80ºC (176ºF) and processing salt, caustic and titanium tetrachloride. The original hot applied PTFE lining required maintenance due to localised disbondment, caused by minor abrasion of the titanium compound. As a result, the plant was facing a long downtime, between two and four weeks, as it was not possible to conduct an in-situ repair.
The Maintenance Manager was keen to keep the downtime to an absolute minimum and ultimately decided to replace the PTFE with a 100% solids modified epoxy novolac system, Belzona 1593, designed for elevated temperature immersion up to 160ºC (320ºF). Belzona 1593 was hand applied onto the reactor in April 2015, facilitating its return to service within only four days. An added benefit, which was considered during material selection, was the servicing of the lining. As this system is applied at ambient temperatures and adheres well to 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 in September 2015 and the maintenance manager was very satisfied with the result. If the lining were to start failing due to poor chemical resistance, this first inspection would have revealed visible signs of degradation. As the lining was found to be fully intact, its durability was not questioned. Moreover, they were able to save time by steam-cleaning the reactor, which was not previously possible with the PTFE lining. They are now looking at replacing the lining on other existing reactors as well as protecting new reactors with the same 100% solids modified epoxy novolac system.
Savings resulting in reduced downtime along with simplified cleaning and maintenance protocols in this case significantly outweighed the initial cost of material.
Belzona 1593, introduced in 2014, is the latest addition to the range of Belzona lining materials, designed for elevated temperature immersion. Incorporation of 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.
The aforementioned example illustrates the difficulties in lining selection for corrosion protection. Simplified approach, which only looks at a few lining features, is not sufficient when requiring a lasting protection. For instance, when specifying protection for a pressure vessel, some of the questions to answer would be:
* What are the design and operating temperatures and pressures of the vessel?
* 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 testing data, both in-house and independent, supported with case studies and other examples of in-service performance?
* How will the lining respond to in-service changes, such as substrate flexing, rapid decompression?
* Can the lining be repaired in situ, can it be cleaned, steamed-out, walked on during inspection? In this case, if localised repair is not possible, a periodical total recoat needs to be factored in the OPEX.
These are just some of the issues to consider and corrosion engineers will have their own lists and guides to follow as well as national and international standards, which 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.