Single-coat inorganic zinc silicate (IOZ) coating provides one of the most durable and cost-effective coating systems available. Properly applied, it will provide many years’ durability in even the most severe environments, such as offshore platforms and other marine applications. It is also widely used in many other industries where color is not required, such as power stations, refineries, bridges and other engineering structures; and water as well as water treatment facilities.
In fact, as long as the environment is not too acidic or alkaline, IOZ can be used in just about any atmospheric application. There are waterborne inorganic zincs, ideal where there are volatile organic compound (VOC) limitations; and solvent-borne coatings, which are easier to use but require humidity to cure. Both types provide similar long-term protection.
Unique Characteristics of IOZ
IOZs have been widely used for tank linings for oils and other hydrocarbons. However, new epoxy and epoxy-phenolic linings have largely taken over, as they are easier to apply and clean, and are more resistant to acidity or alkalinity. (For more about tank lining, read The 5 Most Important Considerations when Selecting Internal Tank Linings.)
Inorganic zinc has a number of outstanding properties compared to other protective coatings:
- It is suitable for high-temp applications up to a dry heat temperature of 750ºF (400ºC)
- It can be used with friction grip joints
- Its toughness makes it effective in bolted applications that would damage other coatings
- It is resistant to most solvents
As with other zinc coatings, IOZ should not be used outside a pH range of about 6 to 12. Nor should it be used for applications where it will be continually wet, such as water tanks or underground.
As a single-coat, thin-film system, it can provide great cost and time savings because blasting and coating can often be done in a day. But with a single coat, you do not have the luxury of second or third coats covering any misses, so thorough visual inspection of every surface is essential. As long as weather conditions are correct, it will rapidly dry and cure, and so can be transported or put into service quicker than many conventional coating systems.
As with other coating systems, IOZs must be applied to surfaces with the correct standard of surface preparation. (Surface preparation is discussed in Substrate Surface Preparation for Corrosion Prevention.) However, IOZ requirements are not too onerous, and getting all of the application stages correct should not prove too difficult for the coatings specifier, contractor and inspector.
Stage 1: Edge Treatment
Treatment of fabrication defects such as rough welds, weld spatter and sharp edges should always be the first stage of any surface preparation, and IOZ systems are no different. However, because IOZs provide galvanic protection and have very little shrinkage on drying, edge treatment can be minimized for these coating systems, cutting costs and time requirements considerably.
Corbett (W. D. Corbett, Modern Steel Construction, June 2000) investigated the corner-build characteristics of common bridge shop primers to determine the required extent of corner preparation, and concluded: “…no treatment of corners is required if an inorganic zinc-rich coating material is specified, provided the coating materials are applied using proper spray technique to ensure full thickness and adequate coverage of the coating”.
According to ISO 8501-3:2006, "Preparation of steel substrates before application of paints and related products – Visual assessment of surface cleanliness – Part 3: Preparation of welds, edges and other areas with surface imperfections", the removal of sharp edges and defects that could cause injury or prevent proper alignment of joints should always be carried out, but treatment of defects to ISO 8501-3 P2 would normally be sufficient.
Stage 2: Oil & Grease Removal
As with any coating system, the next stage will be to ensure that the surface is free from oil and grease. Waterborne versions of IOZs are especially intolerant, so it is good practice to remove any oil and grease using methods described in standards such as SSPC SP1 or AS 1627.1.
Stage 3: Abrasive Blast Cleaning
Inorganic zinc must be applied to an abrasive blast-cleaned surface, with a high standard of cleanliness. The zinc dust needs to electrically connect with the steel surface to provide cathodic protection as the coating chemically reacts with the surface.
A near white metal blast (SSPC SP10/ NACE #2/ ISO Sa 2½) is normally required for atmospheric exposure, although some suppliers allow a slightly lower standard. This would be false economy, and a clean, near-white surface should be mandatory. However, other surface preparation requirements are less critical. A surface profile typically in the range of 40 to 75 microns (1.5 to 3 mils) is often specified, but this is not crucial with IOZs.
For example, the original coating applied to the iconic Morgan-Whyalla pipeline in South Australia, still in good condition 70 years on, was applied to a pickled surface, so steel roughness is clearly not essential. The blast profile should be rough as obtained from angular abrasive but actual profile is not critical.
Stage 4: Salt Removal
The final requirement of surface preparation—freedom from salts—is also less critical with inorganic zincs than for other coatings. As a porous coating chemically adhered to the steel substrate, osmotic blistering is unknown. Moreover, any salt in the air after application can actually assist in the continued curing and hardening of the coating as it assists in polymerization of the silicate.
A surface with heavy salt contamination will rapidly re-rust, so you will lose the blast, but if the surface keeps its near-white metal cleanliness, it should be acceptable for coating with inorganic zinc. (The importance of salt removal is explored in How to Prevent Premature Coating Failures by Removing Soluble Salt Contaminants during Surface Preparation.)
In summary, other than the essential requirement for a surface free from oil and grease with a near-white blast finish, surface preparation requirements for a single-coat inorganic zinc silicate coating system are not too onerous.
The inspector’s main requirement would be for a thorough visual inspection to ensure that all surfaces have been blast-cleaned to the required standard. However, application, drying and curing of these coatings are much more problematic, and these issues will need to be addressed.