Make the right choice for coating fittings.
By Philip Dawson MBA, Technical Advisor, Northpoint Ltd
Metals make it possible to make a wide range of products for use in industry including pipes and pipe fittings for the transportation of fluids and gases like water, oil, and natural gas. Corrosion, however, is a problem. Preventive measures, such as the right coating, can delay or eliminate corrosion.
The annual cost of corrosion worldwide is equivalent to circa 4% of the global gross domestic product, presently more than $3.0/£2.6 T. Historically, corrosion was treated as, ‘break and repair’ issues. Industry 4.0/Quality 4.0 however, has led to preventive measures becoming more significant, systems are designed to reduce corrosion costs. These measures include appropriate material selection, careful component design, and corrosion control.
Active corrosion control involves the use of a sacrificial outer layer (often zinc) that works by corroding at a much slower pace than the underlying substrate. Passive protection involves applying a barrier layer as a coating that prevents corrosive reagents and water reaching the substrate. These coatings may also provide protection against impact, abrasion, and other mechanical damage.
Selecting the best coating technology for an application can be challenging. Here are several guidelines.
Epoxies, the conventional option
Epoxies are widely used as the go-to factory-applied application because they exhibit excellent adhesion to metals and offer high moisture-, chemical-, and some impact-resistance. They’re often used in multi-layer systems, typically 3LPE or 3LPP (3-layer polyethylene and polypropylene accordingly). The epoxy applied is subject to a copolymer layer and then a top layer of the PE/PP. These multi-layer coating systems are often specified where environmental challenges necessitate a thicker layer that may include a higher operating temperature than normal.
Epoxy coatings however do have some limitations driving interest in alternative technologies for corrosion control. Epoxy coatings are not very flexible and may crack in applications that involve land movement, high wear, or heavy impacts. They also become brittle at low temperatures, and yellow/chalk over time in exterior applications due to degradation upon exposure to UV radiation.
Polyurethanes come in several types: solvent-based, two-component (2K) coating systems, 1K water-based PU dispersions, 2K waterborne systems, and high- and 100%-solids 2K formulations.
The 100% solids systems initially presented application difficulties, but advances in application techniques and equipment have overcome these challenges.
PUs exhibit excellent gloss and color retention, combined with good chemical and moisture resistance. Aliphatic PUs also resist UV light for exterior applications. PU coatings mechanical properties include high impact, abrasion, and scratch resistance.
Because polyurethanes comprise two distinct components, their properties are ‘tuneable’, as a result, it’s possible to achieve PU coatings that range from very flexible (elastomeric) to very rigid, that’s not possible to achieve with epoxy systems. PU coatings have excellent adhesion to different substrates, including metal. So, one product line can often be used for multiple applications, reducing costs in inventory. Furthermore, for light- to medium-duty applications, polyurethanes are used as single, direct-to-metal coatings, thereby eliminating the need for a primer, providing for further reductions in operating cost.
Polyurethane coatings cure rapidly, even at lower temperatures, mostly requiring a catalyst. The exception being moisture-cured systems, where the water in the air (humidity) acts as the catalyst.
Most 100%-solids PU coatings require the use of plural component spray technology, which automatically mixes the resin and catalyst components prior to spraying. Operation of this complex equipment requires trained/licensed applicators.
Water Pipelines, Fittings, Structural Steel and Tank Plates
Resicoat R4 Aqua Green – DWI 56-4-659
Eurokote 468 Thixo – DWI 56-4-1209
National Gas Transmission (formerly National Grid) Pipelines and Fittings, listed on NGT QVD Network of approved coatings applicators
Versathane SG – GIS CW6
Macropoxy P100 – GIS CM1/2
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