New water-repellent coating system reduces maintenance requirements

Water-repellent coatings have long been a target for the coatings industry. By actively repelling water from the coated surface, a water-repellent coating can significantly improve corrosion protection, thereby extending maintenance cycles and reducing operational costs for offshore asset owners. Additionally, the water-repellent polyurethane topcoat analysed in this article ensures long-term protection for offshore assets when used as part of a two-coat system, which can be easily applied without special application equipment. There is a significant saving by moving to a two-coat system due to the reduced number of man-hours needed to apply the system compared to conventional three-coat systems, saving asset owners time and expense during asset maintenance and repair work.

The anti-corrosive performance of the water-repellent two-coat system was evaluated through salt spray, cycling ageing resistance, blister box, thermal cycling crack resistance and flexibility tests.

The results show that the water-repellent two-coat system maintains its anti-corrosive properties for extended periods of exposure, making it a suitable for applications in aggressive corrosive environments, such as offshore oil & gas platforms.

The coating system used was:

• Activated zinc epoxy primer
• Water-repellent polyurethane topcoat

For the salt spray, cycling ageing resistance and blister box test, the dry film thickness of system was 280 microns (primer: 140 microns; topcoat: 140 microns). For the thermal cycling crack resistance and flexibility tests, the dry film thickness was increased to simulate over-application.

The water-repellent two-coat system was applied to metal panels. The panels were then exposed to salt spray tests for 6,000 hours, and measurements taken after 1,000, 3,000, 5,000 and 6,000 hours. It is worth nothing that this test goes well beyond a standard salt spray test of 1,440 hours.

The results are shown below. The panels were observed after different times of exposure in order to evaluate the degree of blistering, cracking, rusting and/or flaking. No visible defects were observed after the exposure times. After 6,000 hours of exposure, the corrosion line was less than 1mm, which is quite remarkable taking into consideration the extensive period time of exposure. The adhesion retention was 68% and the final adhesion is acceptable because it is greater than 5Mpa.

An aging cycling resistance test was performed according to the test methods in ISO 12944:2018 Part 9 for CX corrosivity. After the established 4,200 hours, no blistering, cracking, flaking, rust or any other visible defects were observed on the samples and the panels met the test requirements for this environment. Extended testing was also performed following the described method. Up to 7,200 and 10,080 hours, no blistering, cracking, flaking, rust or any other visible defects were observed on the samples.

As expected, the corrosion line increased with the time of exposure; however, rust creep slowed over time and the adhesion properties were not affected. In this test, a standard three-coat system would expect to see adhesion loss of 30-45%. The water-repellent two-coat system vastly outperformed this average; the adhesion properties of the panels maintained approximately 95% of their pre-test values.

A blister box test was performed according to ISO 6270. The water-repellent two-coat system was exposed to the blister box test for 7,200 hours – 10 times the usual test duration. After this period, the panels showed no signs of blistering, rusting, flaking or cracking. The panels showed a decrease of approximately 35% on the POT values when compared to the reference, but these are still regarded as acceptable (above 6 MPa).

A thermal cycling crack resistance test was conducted under the same conditions as NACE standard TM0404- 2004, Section 9, but using L-shape panels instead of C-shaped profiles. The panels were post-cured at 60°C for one week and then subjected to upper and lower temperatures of 60°C to -30°C within a two-hour period for three weeks. At the end of the test period, the coatings were examined for cracking under a microscope, particularly at the corners where over application is normally observed. Neither prototype showed cracks or other defects.

As applicators can have a tendency to over-apply, especially in the corners, the dry film thicknesses of the coating system for this test was 500 microns. This is 2.5 times above the recommended dry film thickness of the water-repellent coating – and the performance indicates that the coating system will still perform extremely well, even if over-applied.

Representative picture of the water-repellent two-coat system following the thermal cycling crack resistance test. The system has a dry film thickness of 500 microns – 2.5 times the recommended dry film thickness of the system – but still shows no signs of cracks or other defects.

One side of the steel panels was coated and cured for two weeks. Afterwards, the test specimens were postcured at 60°C for one week and the respective flexure strain was measured using a fixed-radii mandrel bending machine. The panels were bent over the mandrel with the smallest radius available. No cracks were observed in the coating systems.

The test demonstrates the coating system has a flexural strain of 7%. This is a significant improvement compared to a standard polyurethane coating, which usually has a flexural strain of 1-3%, and shows that the coating is unlikely to crack under pressure or after movement in the steel. This is a very important consideration for coatings used on offshore oil & gas structures, which are often exposed to high temperature fluctuations and movement within the steel.