Examining unit costs and total expenditures is important when selecting a protection system for internal corrosion protection. These additional costs include coverage rate, downtime duration and application – the costs for which can vary based on relative value.
Ultimately, a system that costs less may prove more expensive once total costs are tallied. As an example, this article explains how corrosion-protection selection and maintenance contribute to operating expenditures.
More aggressive corrosive environments persist at elevated temperatures and involve 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 upstream and downstream oil and gas sectors as well as petrochemical, chemical and mineral processing industries. Here, chemicals such as sulfuric acid and hydrochloric acid in combination with seawater and erosive slurries quickly eat away at base metals, typically carbon steels.
Rather than tolerate high corrosion allowances of the base metal, the industry today applies corrosion protection to extend an asset’s life cycle by 20 to 30 years.
Exotic alloys, metal cladding, epoxy and rubber linings are among the materials that protect base metals. The cost is high for exotic alloys and metallic cladding. A rubber lining is at the opposite end of the spectrum. It is relatively inexpensive, but due to its permeability and swelling potential, it is not the most reliable corrosion protection against harsh chemicals.
This article focuses on the middle ground options between the highest and the lowest costs.
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. Its chemical resistance means glass lining can serve many years in environments that quickly render most metal vessels unserviceable.
Glass performs well in a variety of operating conditions, offering strong resistance to corrosion. Its anti-adhesive properties make it suitable for use in the chemical and pharmaceutical industries. At higher temperatures, glass is not as effective against alkalis, where a 18ºF temperature increase doubles the attack rate on glass.
Glass lining costs are directly proportional to its efficacy in extremely aggressive environments. It is also susceptible to impact damage, and its repair is costly. For milder service conditions, a glass-flake technology can be considered.
Glass flakes have been used to improve both barrier properties and reinforcement in anticorrosive coatings since the early 1970s. Strong chemical and erosion resistance means flake-based coatings are used today in various industrial sectors.
How the glass bonds within the different resin matrices is only poorly understood. 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. 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 cost savings 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 inferior to that of an epoxy-based system. Glass flake can be brittle and is easily damaged during inspection or maintenance. The cure mechanism, inhibited by atmospheric oxygen, can lead to significant coating voids that will result in failure. As confirmed by a test sponsored by a global group of energy and petrochemical companies, this is so particularly in decompression situations.
Vinyl ester glass flake systems can therefore be suitable for pipeline protection or storage tanks, but they are not ideal for use in pressurized 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 years. They are continuously modified with the use of new raw materials to improve characteristics that include temperature resistance, abrasion resistance, adhesion and sprayability for ease of application.
First applied to a North Sea platform process vessel in 1987, ceramic filled epoxies today are widely used for erosion-corrosion protection. Limited temperature resistance and sprayability were addressed by introducing modified epoxy novolac, which continuously resist immersion temperatures up to 320ºF, and high molecular weight polymers, which offer superior erosion resistance while being spray-friendly.
Some risks are associated with epoxy coatings use, including applicator error and incorrect coating specification. Both can be addressed with appropriate training and guidance provided by the material manufacturer.
In situations where epoxy coatings are limited in terms of temperature and chemical resistance, polytetrafluoroethylene (PTFE)-based coatings can be used. These coatings are widely used when 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. The durable, low-maintenance alternative to exotic metal alloys offers the thermal stability needed at elevated temperatures. Because PTFE coatings do not react with the process liquids, they prevent contamination.
PTFE is electrochemically, biochemically, enzymatically and chemically virtually inert. Useful properties with no more than 15 percent loss of chemical resistance are retained at up to 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. 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 because it cannot be done in situ.
A polyolefins petrochemical plant based in Ferrara, Italy, was looking for a new coating system for their reactor, operating between 158ºF and 176ºF, 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 was facing between two and four weeks of downtime because it was not possible to conduct an in-situ repair.
The maintenance manager was keen to minimize the downtime and ultimately decided to replace the PTFE with a 100 percent solids modified epoxy novolac system, Belzona 1593, which is designed for elevated temperature immersion up to 320ºF. The system was hand applied onto the reactor in April 2015, which facilitated its return to service within four days. The lining is applied at ambient temperatures and adheres well to metal and itself, so it is repaired in situ when necessary. A similar epoxy system, for instance, has remained in service on a test separator at a North Sea platform for 20 years, and it is patch repaired once every decade during regular inspections.
The reactor was opened for inspection in September 2015. All parties concerned were satisfied with the result. If the lining had started to fail because of poor chemical resistance, this first inspection would have revealed visible signs of degradation. Because the lining was fully intact, its durability was not questioned. Moreover, plant personnel saved time by steam cleaning the reactor, which would not be possible with the PTFE lining. The plant plans to replace linings on other existing reactors as well as protecting new reactors with the same 100 percent solids modified epoxy novolac system.
Reduced downtime along with simplified cleaning and maintenance protocols are leading to savings at the plant that significantly outweigh any premium involved in the initial cost of material. The modified epoxy novolac system, introduced in 2014, is designed for elevated temperature immersion. Incorporation of rubbery domains into the polymer matrix of the material allows the lining to display greater flexibility and creep resistance, improving in-service performance.
The case study example illustrates some of the challenges in selecting a lining for corrosion protection. An approach that is too simple and only looks at a few lining features does not quantify the advantages of lasting protection.
Corrosion engineers will have their own concerns and their own guidelines to follow in tandem with national and international standards addressing 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. The industry will rely on material manufacturers to be efficient and transparent in communicating their advantages and limitations.
Marina Silva is a marketing supervisor with Belzona Polymetrics Ltd. Established in 1952, Belzona designs and manufactures polymer technology, including polymer repair composites and industrial protective coatings. Corporate offices in Europe, North America and Asia support more than 140 distributors in more than 120 countries.