Zinc vs aluminium epoxy primer

[Unserviceable]

Hello all

I've noticed in the shipbuilding world that aluminium epoxy primers seem to be often used on steel hulls, whereas in building construction the primer is almost always zinc epoxy. Does anyone know why aluminium epoxy would be preferred on ships?

 

[hokie66]

Aluminium epoxy mastic coatings, e.g. my old favourite Carbomastic 15, are more forgiving of surface preparation which is not ideal. That may not be the main reason, but it applies.

 

[epoxybot]

I saw this yesterday and was thinking somewhat along the same lines as hokie66 (Structural). I recall reading a bridge specification for a Zinc Epoxy primer for a bridge over sea water and the time window between sand blasting & application was very restrictive. There was considerable concern for flash rusting of the steel deck and the adhesion strength of the Zinc coating, which is finicky in field conditions.

There may also be some formulation advantage in maritime applications to using aluminum vs zinc. If for instance, aluminum particles are less plate shaped than Zinc, they require less resin. Resin is expensive and if you have enough but the formula is too viscous, then you let it down with a diluent. Often, that diluent is Benzyl Alcohol. If water sits on a coating with Benzyl Alcohol as an extender, for long periods; then the coating swells as the Benzyl Alcohols attaches the water molecule. Apart from the Benzyl Alcohol behavior, the rest is pure speculation.

 

[Unserviceable]

Thanks for your replies.

Surface tolerance definitely sounds plausible. While in a shipyard the painting of the blocks would probably be done in a large climate controlled booth, once in service any spot repairs or larger refurbishments are done in dry dock which is essentially open to the atmosphere (and using the cheapest labour possible).

 

[WKTaylor]

Spewin... interesting perspective on finishes over-steel for expensive cargo ships...

Corrosion on the High Seas: How Ship Owners Battle Rust

http://www.materialsperformance.com...-on-the-high-seas-how-ship-owners-battle-rust

The preferred coating system for the ships steel members appears to be...

Base layer... zinc-pigmented [epoxy] primer

intermediate layer... aluminum pigmented epoxy primer

Topcoat layer... Polyurethane [colored barrier coating]

Coatings listed in the article...

"...
Materials selection is one strategy to address corrosion. Corrosion control can be designed into the ship itself. For example, the deck on a tanker can be fitted with ~3 km (2 mi) of piping. Typically, this piping is constructed of stainless steel (SS); however, SS is cathodic to the carbon steel (CS) used on the supports and structure of the ship, which means the CS acts as an anode for the SS piping, and it will corrode preferentially to protect the SS piping. Ideally, the piping material should be designed so the current density from the cathodes (SS) to the anodes (CS) is reduced, which will the prevent the deck of the ship from corroding. Coatings are very good resistors, and on Stolt Tankers ships, the SS is coated to reduce the current density. The CS supports and structure are also coated to protect them from the corrosive environment.

Using high-quality coating systems is another strategy for battling corrosion on the ship. The main deck is protected with a high-grade, combined composite coating system that starts with a zinc primer, followed by an epoxy coating with aluminum pigment as the second coat, and then a topcoat of polyurethane (PUR) coating. Zinc primer is used for adhesion purposes, enabling the coating system to adhere to the metal surface. The epoxy acts as a barrier to protect the metal surface, and reduces the amount of water, chlorides, and other contaminants that can access the surface and cause corrosion problems. The PUR topcoat acts as a sunscreen and protects the epoxy from ultraviolet light. Otherwise, the epoxy would chalk and deteriorate. The PUR is tinted and provides an aesthetically appealing finish to the deck as well.

The epoxy coating with aluminum pigmentation is also used to protect the ballast tanks. Although the International Maritime Organization (IMO) Performance Standard for Protective Coatings2 has a specific objective of achieving a target service life of 15 years for the seawater ballast tank coatings, Stolt Tankers aims to go beyond the standard and meet a 25-year service life of the ballast tanks. To do this, Stolt Tankers protects the CS ballast tanks by applying two coats of this epoxy coating system.

A seawater ballast tank in a Stolt Tankers ship after 20 years in service. Photo courtesy of Stolt Tankers.

Since most of the cargo tanks are constructed of SS, they are typically uncoated. Some cargo tanks, though, are built with CS. In this case, Stolt Tankers lines the CS tanks with either a three-coat epoxy phenolic system comprised of a primer, undercoat, and finish coat, with each coat applied with a dry film thickness (DFT) of 100 µm (4 mils); a three-coat epoxy isocyanate system with a DFT of 90 µm (3.5 mils) for each coat; a zinc silicate coating with a DFT of 80 µm (3 mils); or a two-coat cyclosilicon epoxy system with a DFT 150 µm (6 mils) for each coat.

Are there particular industry standards you follow for corrosion mitigation? How do these standards help with corrosion mitigation?

JE/MR: The industry standards available for corrosion provide guidelines for anticorrosion design during a vessel’s planning and construction stages, as well as protocols to follow for maintaining corrosion protection systems during the life of the ship. There are a number of standards used. For example, maintenance standards for coatings cover areas such as surface preparation, coating application, coating inspection, and how to determine coating deterioration. The standards clarify what is required and make it possible for all parties involved with ship construction and maintenance to have a comparable understanding of the requirements. The most common standards used are the four parts of ISO 8501, “Preparation of Steel Substrates Before Application of Paints and Related Products—Visual Assessment of Surface Cleanliness,” and multiple parts of ISO 4628, “Paints and Varnishes—Evaluation of Degradation of Coatings—Designation of Quantity and Size of Defects, and of Intensity of Uniform Changes in Appearance.” Without standards, we wouldn’t be able to build or maintain a ship.
..."

Regards, Wil Taylor

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