Coating Properties And Test Procedures

The coating is the premier external corrosion protection method for pipelines. As such, the selection and continued integrity of any parent or rehabilitation pipeline coating is of paramount importance. It is the coating that determines cathodic protection (CP) requirements and the magnitude of pipeline maintenance expenditures to prevent failures such as metal loss and stress corrosion cracking. The fundamental contributions that the coating makes to stopping external corrosion are:

1. Isolating the anode and cathode areas of the corrosion cells from each other.
2. Isolating the common electrolyte - the moist soil - from both the anode and cathode areas.

These two inter-dependent activities identify water resistance as the premier property of a coating. However, the absorption of water - which is strongly influenced by temperature - can significantly influence the coating’s mechanical (secondary) properties such as soil stress resistance, adhesion and impact resistance. In addition, there is a third level of properties related to coating composition such as wetting ability, coating filler and solvent content.

Identifying a suitable coating from the supplier’s literature is complex because the properties quoted predominately relate to mechanical properties. No mention is made as to how these mechanical properties degrade as the coating absorbs water. Mechanical properties are important when building a pipeline. Good abrasion and impact resistance are desirable to reduce construction and stone damage. Unfortunately, if the coating - through handling damage - then absorbs water when buried, it can soften such that the slightest movement can cause significantly damage (Figure 1).

By understanding how parent coatings fail, we could improve the way parent and rehabilitation coatings are selected. To improve understanding, the basic electrochemical principles of several test procedures used to evaluate thin film coatings applied as a liquid or fused powder are reviewed.

Consider the major failure mechanism of thin and thick film coatings. Thin film coatings of urethane, urethane tar, liquid epoxy, polyester and FBE are typically applied 400-1,000 μ thick. The predominant failure mechanism of thin film coatings is by blister formation through water absorption. All coatings absorb a small amount of water. As an example, freshly applied fusion bonded epoxy (FBE) contains about 0.25 w/o moisture. The detrimental effects of moisture are shown during field-joint coating.

Induction heating applied to raise the temperature to approximately 250oC converts any water to steam, resulting in a big volume expansion. From basic chemistry, 18 cc of water, when converted to steam at the same temperature and pressure, occupies 22.4 litres. As the temperature is raised, the FBE softens, and at temperatures above 220oC, the strength of the FBE and its adhesion to the steel fails as the coating blisters. The only way to prevent blistering is to preheat to 125oC so the coating still has strength, then allow time for the steam generated to diffuse out of the coating. This typically takes about 15 minutes.