Inorganic zinc primers, while requiring more care in surface preparation and application compared to organic zinc primers, have a greater ability to withstand exposure to most solvents, oils, and neutral petroleum products. Some types of inorganic zinc primers require moisture to cure. Inorganic zinc primers have a tendency to mudcrack at heavier thicknesses, and dry spray may occur under certain ambient conditions. They function well up to 400 °C (750 °F) in dry conditions. In wet conditions, inorganic zinc-rich primers function well up to 50 °C (120 °F). Within the range of 50°C to 60°C (120 °F to 140 °F), performance depends upon formulation and the coating manufacturer should be consulted. In wet conditions above 60 °C (140 °F), zinc-rich primers are not recommended. Compared to organic zinc primers, inorganic zinc-rich primers generally exhibit more pinholing and bubbling upon topcoating.
Inorganic zinc-rich coatings usually have silicate resins that may be water-borne or solvent-borne. These coatings form a film that provides galvanic protection of the underlying steel. The zinc metal in the film is preferentially corroded to protect iron and steel because it is higher in the galvanic series. A high concentration of zinc particles in the film will provide the necessary conductivity for galvanic protection. This high zinc loading contributes to the film’s porosity and its poor internal cohesion.
A two component, rapid recoat, fast curing solvent based inorganic zinc rich ethyl silicate primer, containing 85% zinc by weight in the dry film. Conforms to SSPC Paint 20 Level 1. The major catagories o inorganic zinc-rich coatings are water-borne and solvent-borne. The classification scheme is derived from SSPC-Paint 20, which includes both Type 1 (Inorganic) and Type 2 (Organic). Some water-borne inorganic zinc coatings are cured after application by heat or an acid curing agent. Most, however are self curing. They simple react with carbondioxide from the air (Type 1B) Solvent-borne alkyl silicate inorganic zinc rich coatings are self curing and depend on moisture in the air to complete the hydrolysis reaction(Type 1C). When the weather is hot and dry, it may be necessary to spray water on these coatings to complete the curing.
All surfaces to be coated should be clean, dry and free from contamination. Prior to paint application, all surfaces should be assessed and treated in accordance with ISO 8504:2000. Oil or grease should be removed in accordance with SSPC-SP1 solvent cleaning.
Abrasive blast clean to a minimum of SSPC-SP6 or Sa2½ (ISO 8501-1:2007), (or SSPC-SP10 for optimum performance). If oxidation has occurred between blasting and application of inorganic zinc, the surface should be reblasted to the specified visual standard. Surface defects revealed by the blast cleaning process, should be ground, filled, or treated in the appropriate manner. A surface profile of 1.6-3.0 mils (40-75 microns) is recommended. If the zinc shop primer shows extensive or widely scattered breakdown, or excessive zinc corrosion products, overall sweep blasting will be necessary. Other types of shop primer are not suitable for overcoating and will require complete removal by abrasive blast cleaning. Weld seams and damaged areas should be blast cleaned to SSPC-SP6 or Sa2½ (ISO 8501-1:2007).
All damaged areas should ideally be blast cleaned to SSPC SP6 or Sa2½ (ISO 8501:2007). However, it is acceptable that small areas can be power tool cleaned to SSPC SP11 or Pt3 (JSRA SPSS:1984), provided the area is not polished. Repair of the damaged area can then be carried out using a recommended zinc epoxy primer.
Inorganic zinc is supplied in two parts, a liquid Binder base component (Part A) and a Powder component (Part B). The Powder (Part B) should be slowly added to the liquid Binder(Part A) whilst stirring with a mechanical agitator. DO NOT ADD LIQUID TO POWDER. Materialshould be filtered prior to application and should be constantly agitated in the pot duringspraying. Once the unit has been mixed it should be used within the working pot life specified.Do not allow material to remain in hoses, guns or spray equipment. Once unitsof paint have been mixed they should not be resealed and it is advised that after prolongedstoppages work recommences with freshly mixed units.Clean all equipment immediately after use. It is good working practice to periodically flush out spray equipment during the course of the working day. Frequency of cleaning will depend uponamount sprayed, temperature and elapsed time, including any delays.All surplus materials and empty containers should be disposed of in accordance withappropriate regional regulations/legislation.
Prior to overcoating, inorganic zinc must be clean, dry and free from both soluble salts and excessive zinc corrosion products. Surface temperature must always be a minimum of 5°F (3°C) above dew point. When applying inorganic zinc in confined spaces, ensure adequate ventilation. The minimum overcoating interval is dependent upon the relative humidity during cure. If thinning is required to assist spray application in warmer climates, (typically >82°F (28°C)), it is recommended that thinners are used. It is recommended that prior to overcoating a solvent rub test to ASTM D4752 should be undertaken. A value of 4 indicates a satisfactory degree of cure for overcoating purposes. At relative humidities below 55%, curing will be retarded. Humidity may be increased by the use of steam or water spraying. Excessive film thickness and/or over-application of inorganic zinc can lead to mudcracking, which will require complete removal of the affected areas by abrasive blasting and re-application in accordance with the original specification. Care should be exercised to avoid the application of dry film thicknesses in excess of 5.0 mils (125 microns). For high temperature systems the thickness of inorganic zinc should be restricted to 2 mils (50 microns) d.f.t. Continuous dry temperature resistance of inorganic zinc is 752°F (400°C) if left untopcoated
*Can be low in VOCs
*Excellent heat resistance
*Excellent abrasion resistance
*Good atmospheric durability
*Useful as shop primer
*Can be used untopcoated
*Needs very clean, blasted surface
*Requires skilled applicator, agitated coating
*Difficult to topcoat
*Attacked by acid and alkali
*High initial cost
1.Rust Bullet, Zinc-Rich Coating Systems , Date of access: 27 October 2016, http://www.rustbullet.com.au/technical/sspc-the-society-for-protective-coatings/painting-systems/guide-to-zinc-rich-coating-systems/
2.Materials Performance, What Do You Know about Inorganic Zinc Coatings?, Date of access: 27 October 2016, http://www.materialsperformance.com/articles/coating-linings/2016/06/what-do-you-know-about-inorganic-zinc-coatings
3.International, Inorganic Zinc-Rich Silicate, Date of access: 27 October 2016, http://www.international-pc.com/pds/4518-p-eng-usa-ltr.pdf