The ocean is the place with the worst corrosive environment.
The corrosion area of the offshore platform can be divided into: A. marine atmosphere B. splash area C. tidal range D. full immersion area E. sea mud area
The marine coating system includes primers, intermediate coatings and topcoats, and the underwater parts also include antifouling paints.
Primer is based on high anti-corrosion primer. Commonly used primers include epoxy zinc-rich primer, inorganic zinc silicate primer, and epoxy anti-rust primer. Zinc rich primer is an organic or inorganic coating containing a high proportion of zinc powder as a filler. The zinc rich coatings use high-build epoxy coating with a volume solid content of more than 70%, strong adhesion, and surface tolerance.
The function of the intermediate coat and topcoat is to provide a protective layer for the primer, slow down and limit the penetration of water vapor, oxygen and chemically active ions. Commonly used topcoats in the marine offshore platforms include aliphatic polyurethane paint and polysiloxane coatings. Aliphatic polyurethane coatings are often used in conjunction with zinc-rich primers and high build epoxy intermediate coat. The polysiloxane coating is directly matched with the zinc-rich primer coating. Epoxy polysiloxane coatings have stronger corrosion resistance, and acrylic polysiloxane coatings are better in decoration property.
1. Surface treatment
Preparing the metal surface before coating is an important requirement. Among the factors that affect the effectiveness of coating performance, surface cleanliness may be the most important. Any scale, dirt, grease and rust must be completely removed before painting to ensure that the coating film is completely and firmly attached to the substrate. If the adhesion is insufficient, the barrier protection will fail.
The cleaning techniques used include pickling or shot blasting. Pickling is used for metals coated by a galvanizing process. (Pickling is discussed in the article Using pickling and passivation chemical treatments to prevent corrosion.) For offshore structures and ships, sandblasting is the prescribed cleaning method. For structural metals such as steel and alloy steel, abrasive cleaning methods are also used for surface treatment.
2. Choosing the right protective coating
For surfaces that remain fully submerged, it is best to use barrier coating systems and cathodic protection. Compared with metal spraying, spray paint has always been considered the first choice for corrosion protection. Here, paint is used in multiple layers to enhance corrosion protection. (Learn about polyurea coatings in flexible coatings used to protect marine structures.)
Paint consists of pigments, binders and solvents. The pigment that constitutes the core of the coating consists of solid particles, which are kept well dispersed by solvents and binders. Coatings can be specially formulated to ensure the basic properties required for specific applications, such as water resistance and scratch resistance. In order to minimize coating defects, apply multiple layers of paint on the substrate. Before applying the first coat, apply a primer on the substrate to improve adhesion.
3. Marine environmental protection coating
Impressed current systems cannot protect surfaces and waterline areas exposed to the atmosphere, because splashing water can cause severe corrosion. The surface of the waterline should be coated with the thickest and best epoxy resin. Cathodic protection can protect the underwater hull, which is the area most prone to corrosion deterioration and fluid turbulence.