Damage Mechanisms Affecting Fixed Equipment in the Refining Industry

API RECOMMENDED PRACTICE 571

1- Mechanical and Metallurgical Failure Mechanisms

2- Uniform or Localized Loss of Thickness

3- High Temperature Corrosion [>400°F (204°C)]

4- Environment – Assisted Cracking

Repairing Damaged Areas of Galvanized Steel

ASTM A 780 authorizes three accepted touch-up and repair methods

1- Applying zinc-rich paint: According to ASTM A 780, organic or inorganic zinc-rich paint for touching up and/or repairing galvanized steel must contain between 65-69% zinc by weight, or >92% by weight metallic zinc in dry film. Inorganic zinc-rich paints are more effective than organic in terms of delivering corrosion protection, and they do not shrink after drying/curing as organic coatings do. This is a major advantage when the paint is applied to corners, edges and rough surfaces.

2- Coating with zinc solders: Soldering with zinc-based alloys consists of applying zinc alloy -in stick or powder form -to the area to be repaired, which has been preheated to approximately 600 F (315 C). Common repair solders include zinc-tin-lead, zinc-cadmium and zinc-tin-copper alloys.

3- Spraying with molten zinc (metalizing): Metalizing is the melting of zinc powder or zinc wire in a flame or electric arc and projecting the molten zinc droplets by air or gas onto the surface to be coated. The zinc used is nominally 99.5% pure or better. The performance of wire versus powder is comparable. Zinc-aluminum alloys also can be used. The application equipment used may limit the concentration of aluminum.

Paints: Inorganic Zink vs Organic Zinnk

Zinc-rich coatings and primers have the unique ability to provide galvanic protection to the steel surfaces to which they are applied. These coatings have a large amount of metallic zinc dust combined with the binder. 

There are two main types of zinc-rich coatings, which differ in type of binder. 

1- Inorganic zinc-rich coatings generally have a zinc silicate binder while the organic variety uses an organic resin such as an epoxy, butyl, or urethane. After proper application of a zinc-rich coating to a steel substrate the binder holds the zinc particles in contact with each other and the steel surface. This contact between two dissimilar metals, when in the presence of an electrolyte, will form a galvanic cell. The zinc particles become the anode in the galvanic cell and the steel substrate serves as the cathode. 

Galvanic action causes the zinc to be preferentially corroded while the steel is protected from attack. Zinc-rich coatings are unique in that they provide protection to the steel surface even at voids, scratches, pinholes and other small defects in the coating system.

2- Organic Zinc-Rich Coatings: Organic zinc-rich coatings are generally formulated from epoxy poly-amide, vinyl, urethane, and chlorinated rubber binders. The type of binder used ultimately determines the drying and curing of the organic zinc-rich coating. These coatings use zinc dust as a pigment in high concentrations to achieve a dry film pigments volume of 75 percent or higher. When zinc particles are formulated into organic vehicles, the binder more thoroughly encapsulates the zinc particles than with inorganic vehicles.

This encapsulation somewhat reduces the sacrificial capabilities of the applied coating. However, this characteristic of the binder also allows the coatings to more readily wet and seal the prepared surfaces. In this way, organic zinc-rich coatings are more tolerant of incomplete surface preparation. Top coating with the same generic type of organic topcoat is more easily accomplished than with inorganic zinc-rich coatings because of a less porous surface. 

Organic zinc-rich coatings are often used to touch up and repair inorganic zinc-rich coatings because the organic binder provides better adhesion and wetting characteristics than another coat of inorganic zinc-rich coating. 

Organic zinc-rich coatings don't provide the same heat and abrasion resistance of the inorganic zinc-rich coatings. Organic-zinc rich coatings are generally considered easier to apply and topcoat than their inorganic counterparts, however, they do not provide the same long-term corrosion protection as the inorganic zinc-rich coatings.