A brief discussion on the types and selection of rust-primer coatings for metal surfaces

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This post was last edited by Wang Wei2 on 2023-8-23 at 10:45. A brief discussion on the selection of primers for metal surfaces with rust: 1. Types of primers for rusty surfaces. Depending on the mechanism by which they interact with rust deposits, rust-resistant coatings can be divided into three types: conversion type, stabilization type, and penetration type. See Table 1 for details. Table 1 Types of Rust-Primer Coatings Type Mechanism Application Conditions Advantages Disadvantages Transformation coatings: Also known as reactive rust-primer coatings, these involve the use of phosphoric acid combined with xanthate or tannic acid as transforming agents – substances that can undergo a chemical reaction with rust to convert it into harmless complexes or chelates that provide some level of protection. These agents are added to the coating mixture and applied to rusty steel surfaces to form a coherent protective layer with good adhesion. Suitable for steel surfaces with a relatively uniform rust layer, free of residual scale flakes, thick rust, and old paint, where the rust layer is thick. It can convert severe rust, with a fast conversion rate. It is difficult to determine the relationship between the thickness of the rust layer and the amount of paint required. If the acid content is low, the rust does not convert completely; if the acid content is high, acidic substances remain in the paint film, which can corrode the steel. Such paint cannot serve as a proper primer and can only be used as a surface treatment agent. To achieve corrosion protection, acid-resistant primers and topcoats must be applied additionally during application. Stable rust-coating paints achieve the effect of rust-coated finishing by adding various rust agents and stabilizers to the film-forming substance. It can be used on steel surfaces with uneven rust, residual hard oxide scales, or old paint, and can be processed by fusion welding. It can be used directly as a primer in combination with other topcoats. The downside is that it can only be used on steel surfaces with light rust (a rust layer thickness of less than 30 micrometers). Penetrating rust-resistant coatings utilize the wetting and penetrating properties of the paint to separate the loose rust and enclose it within the paint, thereby preventing further corrosion. Suitable for application on steel surfaces that have been corroded for a long time and are minimally chemically contaminated; it does not contain rust stabilizers or conversion agents, and thus does not corrode the steel. It provides excellent adhesion to metals as well as corrosion protection. Applicable to rust products of 50 microns and above. 2. Screening of rust-primer systems: Analysis of the various rust-primer systems available in China shows that conversion-type primer coatings have a corrosive effect on metal surfaces ； Stable rust-resistant coating, suitable only for rust removal on surfaces smaller than 30 microns. Neither of these primers is suitable for surfaces that already have an old coating. Penetrating rust-resistant coatings do not corrode metals; they offer good adhesion to existing coatings and can handle thick layers of rust deposits. 2.1 Penetrating rust-inhibiting coatings: Unlike other types of rust-inhibiting coatings, penetrating rust-inhibiting coatings do not undergo any chemical reaction with rust; rather, they rely on mechanical physical penetration and encapsulation to isolate the rust from external corrosive agents, thereby preventing it from becoming active and continuing to cause further rust formation. Key components: Penetrant: Its function is to mechanically loosen the rust, allowing the adhesive to penetrate more effectively into the rusted area and bond with the surface of the substrate. Adhesive: That is, the resin matrix, which primarily physically encloses the softened rust, isolating it from external substances and enabling it to adhere to the surface of the metal substrate. The penetration depth of rust caused by adhesives is related to the following factors: ① Lower viscosity results in greater penetration depth ； ②As the coating thickness increases, the penetration depth also increases ； ③If the coating dries quickly, its penetration is poor ； ④When the rust structure on the metal matrix surface is coarse and porous, the penetration depth is good; whereas when it is fine and porous, the penetrability is poor. 2.2 Function of Penetrating Rust-Removing Coatings: The basic principle of penetrating low-pretreatment coatings is to use resins with strong penetration properties as the paint base; together with penetrants and polar solvents, this enables the coating to have good penetration capabilities, allowing it to soak into and penetrate the rust layer on the steel surface and seal it off. It can effectively isolate rust and external corrosive factors, enhancing the adhesion between the rust layer and the steel plate, between the rust layer and the coating, and between the coating and the steel plate. These substances can protect metal surfaces through principles such as \"adsorption\" and \"film formation\". Through the coated surface after penetration, the penetrant has tightly enclosed the rust products on the metal, forming an integral part of the anti-corrosion coating and thus serving its anti-corrosion function. In actual metal surface coating, due to constraints in the application site, methods such as mechanical sandblasting for rust removal cannot be used; it is recommended to prioritize the use of penetrative rust-prone coatings. Coating can be applied to metal surfaces with rust of 50 microns or less, as well as to those with existing paint coats that have good adhesion; this is particularly advantageous for equipment that requires secondary repairs and for the metal surfaces of steel structures.