Protective coatings for steel piling include, among others, coal tar epoxy, fusion-bonded epoxy, zinc coatings and powder coatings. Of these, only zinc and powder coatings are allowed for corrosion protection of helical piles, as outlined within the International Code Council (ICC) Acceptance Criteria (AC) for Helical Pile Systems and Devices (AC358). AC358 specifies that zinc coatings comply with ASTM A123/A153, B633 or B695 for hot-dip galvanizing (HDG), electrodeposited zinc or mechanically-deposited zinc, respectively. The powder-coated finish allowed in AC358 must be a polymer ethylene and acrylic acid (EAA) and comply with ICC AC228 (Acceptance Criteria for Corrosion Protection of Steel Foundation Systems Using Polymer (EAA) Coatings), with an exception that the coating thickness be at least 18 mils (0.018 inch) versus the 10 mil ± 2 mil thickness allowed in AC228.
Hot-dip galvanizing is by far the preferred method of both helical pile manufacturers and design professionals for protecting helical pile components from corrosion. That said, there remains some confusion about the use of powder coating, how it compares to HDG and, ultimately, its effectiveness to provide the corrosion resistance advertised.
Processes:
The process of hot-dip galvanizing starts with dipping the steel in a caustic solution of high alkalinity to remove organic residues. The part is then rinsed with water and put into a pickling bath of hydrochloric or sulfuric acid. The pickling removes surface oxides and mill scale from the steel. After pickling, the part is rinsed in water again and put into a molten zinc bath with flux to fuse zinc to the parent material. Finally, the part is inspected for surface imperfections and coating thickness.
Polymer powder is made from polymer resin that is combined with curatives, pigments, leveling agents, flow modifiers and other additives that are melted, mixed and then ground into a powder form after cooling. The powder coating process with EAA copolymers involves surface preparation of the steel to remove contaminants followed by application of the powder via a spray gun. The steel part is electrically grounded and the spray gun applies an electrostatic charge to the powder which creates an electrical attraction between the powder and the steel surface. After the powder coating is applied, the part is put into a curing oven to complete the bonding and setting process.
Fundamental Difference:
A fundamental difference between zinc and powder coating is the way each protects the base metal from corrosion. Zinc coatings are considered sacrificial elements where the fused zinc layer is depleted during the corrosion process. Corrosion is an electrochemical process where the zinc acts as an anode, the base steel is the cathode, and the corrosive soil (and moisture) is the electrolyte that completes the electrical circuit. The anode (zinc coating) has a more active voltage than the base metal which creates a difference in potential between the two, allowing the zinc to corrode in preference to the base metal. Even in situations where the zinc coating has been damaged, exposing the bare steel, the bare steel will not corrode until all of the electrically-connected zinc is consumed.
On the other hand, powder coatings are simply inert, protective barriers on the surface of the base metal. Therefore, if the powder coating is compromised during product storage, transport or installation, the exposed area will immediately begin to rust.
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