Finding a Better Tank Lining to Resist Aggressive Acids

When one of the world’s largest providers of products and services in the energy field experienced lining failures in tanks holding an especially aggressive acid, the company asked Sherwin-Williams Protective & Marine for assistance. Thankfully, the partnership – which also included the maker of the tanks – led to a solution that is helping the tanks and coatings industry better protect tanks from the aggressive acid.

The tanks were in the Gulf of Mexico, where drilling and oil well processing were taking place. The aggressive acid was needed to help open cavities in rock formations and keep wells free of clogs over time. Unfortunately, this acid can also corrode vessels in which it’s contained, so a strong tank lining is vital. The coating that was failing in the tanks was a vinyl ester system, which seemed appropriate, but the coating was blistering and lifting from the substrate nonetheless.

Through lab testing, Sherwin-Williams determined that its Magnalux™ 2200GF coating, a vinyl ester, graphite-filled novolac, was by far the best option for the tanks holding the aggressive acid. The test results impressed not only the oil producer but also the tank supplier, which now applies Magnalux 2200GF to its entire fleet of severe service tanks, used by customers all over the world.

An Aggressive Chemical

The aggressive acid stored in the tanks is used to help drill through salt formations and reach hydrocarbons below. In fact, the acid is designed specifically to eat through formations containing sodium chloride and potassium.

The acid also keeps wells open. Offshore wells can last 25-30 years, and during that time, the cavity can narrow as various materials, like lubricants and salt, cake up inside. This reduces the well’s capacity and production. Acid injected into a well dissolves that built-up material.

Storing and moving the aggressive acid requires caution. The material is transported to a dock, transferred into steel tanks or common carrying units (CCUs), placed on a vessel or barge and shipped to the offshore drilling site. The tanks are designed to handle the aggressive acid, and the lining used must protect the tank interiors.

Vinyl Ester Mystery

The most effective linings for vessels storing this aggressive acid are vinyl esters, a cross between polyester and epoxy resins that are stronger than both. Vinyl esters are highly resistant to corrosion and acidic commodities. The oil producer’s tanks were lined with vinyl esters made by another coatings manufacturer. However, the linings were inexplicably failing within six to 12 months of going into service. In addition to the coating blistering and lifting from the substrate, deep corrosion pits appeared in the steel.

The oil company and tank manufacturer shared photos of the failing coatings with Sherwin-Williams Protective & Marine and provided a safety data sheet for the acidic solution stored inside the tanks, which allowed lab technicians to analyze its composition. Before long, the technicians spotted the flaw. The aggressive acid includes hydrofluoric acid (HF), which attacks anything that is silica-based, and the other manufacturer’s coating was reinforced with a glass-flake filler pigment. Therefore, the glass flake in the coating material that was meant to increase protection was instead reducing the lining’s performance. So, despite the coating having a vinyl ester backbone that should have enabled long-term lining performance, the coating’s glass-flake filler was actually hindering its effectiveness.

The Test

To consider a viable alternative to the glass flake-filled vinyl ester lining, Sherwin-Williams created an ad-hoc three-month testing protocol for the aggressive acid and various coating samples. In an autoclave test, lab technicians immersed coated panels in the acid and heated the immersion bath to 120°F (49°C). The fully-immersed panels were then subjected to three months of immersion at a constant temperature of 120°F during the autoclave test. Lab technicians looked for visual signs of failure such as blistering and cracking in the coating.

To test coating adhesion, technicians glued metal dollies to coated substrates – after the coatings cured for seven days in ambient conditions – and then attempted to pull off the coating using standard test methods (Figure 1). The adhesion value was the pounds per square inch (psi) value needed to remove the dollies from the substrates. The mode of failure was very important in determining the best option moving forward since the original coating that was failing was lifting from the substrate in the tanks. Technicians also checked the weight of the coating samples over three months to determine how much of the tank commodity had permeated the coating.

Four types of coatings, all Sherwin-Williams products, were tested. One was a glass flake-filled coating that was similar to the other manufacturer’s failed lining. Two systems were carbon-woven laminates, which were identical in chemistry but were applied slightly differently. With both laminates, a primer and resin were applied before the carbon-woven laminate material, followed by a gel coat to seal the system. This top layer literally seals the system, eliminating any wicking fibers from the carbon-woven material and providing additional barrier protection between the laminate and the vessel contents.

The fourth coating tested was similar to the other manufacturer’s lining that was in the original tanks, but instead of containing a glass-flake filler, the coating – Magnalux 2200GF – was graphite filled. Lab technicians suspected going into the test that Magnalux 2200GF would be the best option.

The Results

The glass flake-filled coating performed as expected. After two months, it showed signs of weakening, and at three months, it was removed from the substrate at relatively low pressures when adhesion tests were performed. The single-layer laminate system was inconsistent and scored low adhesion values over three months, while the double-layer laminate provided reliable protection even though its adhesion values declined over time. The two laminate systems were legitimate subjects for the test because they normally perform well in downstream refinery tanks and secondary containment areas holding aggressive chemicals. However, laminates can be challenging to install, increasing the likelihood of application error. Laminates were simply not the ideal choice in this case.

On the other hand, Magnalux 2200GF provided extremely consistent protection from the aggressive acid throughout the three-month test (Figure 2). The lining’s adhesion numbers were excellent. Furthermore, the product is easy to apply compared with laminates, requiring only a single-leg airless spray pump.

Figure 3. The tank manufacturer is now installing the graphite-filled Magnalux 2200GF lining in tanks that will house the aggressive acid to enable greater corrosion protection and longer tank service lives

Figure 1. The coating adhesion test involved pulling glued metal dollies from coated panels to measure the pull-off strength in pounds per square inch (psi). The panels featured (from left to right) a glass flake-filled coating, a carbon-woven laminate system, the graphite-filled Magnalux 2200GF product and another carbon-woven laminate system.

Figure 2 (below). Following one, two and three months of immersion in a highly aggressive acid, the glass flake-filled coating (far left) significantly weakened and deteriorated. The single-layer laminate system (far right) scored low adhesion values, while the double-layer laminate (second from the left) performed better despite some loss of adhesion. The Magnalux 2200GF coating (third from the left) had excellent adhesion numbers and provided consistent protection from the acid throughout the three-month test.

One Month

Two Months

Three Months