Pitting repairs in HT/HP process vessels

Crevice and pitting corrosion represent the main types of localised corrosion.

The costs of corrosion can be colossal, especially where safety critical equipment is concerned, and especially in the oil, gas and petrochemical industries. There are direct costs involving equipment and part replacement, while hidden costs include downtime, delays, litigation and other unplanned overheads.

The most damaging form of corrosion is localised corrosion, which does not proceed uniformly and is focused at particular sites on a steel substrate. Crevice and pitting corrosion represent the main types of localised corrosion.

In pitting corrosion, an anode develops and maintains its electrical potential with respect to the surrounding metal, with a large cathode to anode ratio that allows the corrosion to rapidly form a pit. Pitting corrosion is especially prevalent in steels that have the ability to passivate – especially in stagnant conditions where the formation of a protective film is hindered by the presence of chloride ions. It is considered more dangerous than uniform corrosion because it is more difficult to detect, predict and design against. It is also difficult to repair.

Pitting can be prevented and controlled by using corrosion inhibitors, cathodic protection, and protective coatings, but these protective systems have been known to fail. Once pitting occurs a solution is needed that can satisfy three basic needs: (1) quick repair, (2) ease of repair, and (3) rapid return to service. Additionally, the maintenance solution should withstand service conditions for a considerable time.

Localised corrosion in the form of deep pits can be weld repaired to restore the original profile, but expertise and special tools are required. If either is lacking, repairs can do more harm than good because of the risks of distortion, weld cracks, stress corrosion and health & safety considerations. Welding repairs carried out on metal substrates over 30mm thick must also involve post weld heat treatment (PWHT,) which may result in the loss of weld metal strength and toughness. PWHT is also costly because of the time that it takes – up to 40 hours. Further, welding over a metallic substrate involves metal being applied to metal, which does not remove the original problem unless the metallic substrate is coated with an organic protective material.

Another viable option to repair pitting corrosion is the use of cold applied epoxy materials. These 100% solids paste grade materials have been on the market since the 1960s and have been continuously improved to withstand greater temperature and pressure levels as well as various in-service conditions. One example from the UK is that of an amine reboiler vessel at a gas terminal, which suffered heavy pitting corrosion discovered in 2011. The operator required the vessel to be back in service as soon as possible and was looking for an alternative solution to hot work. A paste grade epoxy material was chosen to fill the pits and the wall was protected with a modified epoxy novolac coating afterwards. Both the coating and paste grade material were designed to achieve full curing in high-temperature immersion service, minimising downtime. The reboiler was opened up for inspection in July 2015. No further pitting damage or corrosion was identified. Minor localised repairs were completed on the coating and the reboiler was returned to service.

In order to ensure fitness for service of pit-filling epoxy paste grade materials, the application should be carried out in strict accordance with manufacturer’s requirements. The contracting company must ensure that the surface is prepared correctly, that the repair material is mixed and applied properly and that it is allowed to cure in accordance with manufacturer’s instructions. A typical pit filling procedure is summarised as follows.

All work must be carried out in accordance with the manufacturer’s instructions. The vessel substrate must be dry and contaminant-free.

Sharp edges or irregular protrusions should be ground down to a smooth contour with a radius of not less than 0.1 inch (3 mm.) All surfaces must then be grit blasted using an angular abrasive to Swedish Standard SA 2 ½ (near white metal finish) with a minimum profile of 3 mils (75 microns.)

Paste grade epoxy material must be mixed in the correct ratio.

The material needs to be applied onto the substrate until original wall thickness is restored.

The material must be allowed to solidify at ambient temperatures before achieving full cure in service.

One drawback traditionally associated with the use of epoxy materials for pitting repairs has recently been overcome, namely the amine bloom film which would appear on the surface during cure. The bloom would manifest in the form of sticky deposits that affected overcoatability and intercoat adhesion. It had to be removed by first washing with a hot detergent solution followed by a fresh water wash, and then frost blasting prior to the application of a protective coating atop the pitting repair, leading to extended application time and labour costs.

The latest innovation in raw materials has brought on non-bloom technology, where frost blasting of the applied material prior to the application of protective lining is not required. This feature was incorporated into the reformulated version of the Belzona 1511 (Super HT-Metal,) which has been on the market since 2001. In addition to incorporating non-bloom technology, further evaluation revealed the following enhanced features:

Frost blasting of the Belzona 1511 is no longer required when a protective lining is being applied atop with a 24-hour overcoat window, thus reducing application costs. Application is also simplified with mixing and application possible at temperatures as low as 10°C (50°F.)

The rubbery domains used in Belzona 1523 and Belzona 1593, which were also incorporated in the polymer matrix of Belzona 1511, have improved the adhesion, flexibility and toughness of Belzona 1511. Tensile shear adhesion (ASTM D1002) has increased by 46% regardless of the cure temperature. Pull off adhesion has increased by 34% (ASTM D4541/ ISO 4624.)

Continuous advancements in raw materials make it possible for coating and composite manufacturers to produce systems that are better value and easier to apply, at the same time minimising the risks typically associated with hot work. In this way, the indirect costs of corrosion, including downtime, delays, litigation and other unplanned overheads, can be significantly reduced.