KTA-Tator, Inc.

About Us

Services

Careers

Trade Shows

News & Articles

KTA-TATOR, INC.
Corporate Headquarters
115 Technology Drive
Pittsburgh, PA 15275
Phone: 412.788.1300
Fax: 412.788.1306
E-mail: info@kta.com

Instrument Sales
1-800-KTA-GAGE
Lead Hotline
1-800-LEAD-OFF

Home

KTA Challenge #13
Answer - Inorganic Zinc


Photograph 1	Photograph 2

When properly applied, an inorganic zinc rich primer creates a very tough, abrasion resistant and corrosion resistant film, but because of the heavy zinc loading, its cohesive strength is not the same as that found in other coatings, such as epoxy or urethane. When conducting knife adhesion tests of an inorganic zinc/epoxy/urethane system it is not uncommon for forced failure to occur within the zinc primer. While such a result does not necessarily mean that the system is deficient or that it will not perform, in this case, the adhesion was abnormally poor. In fact, it was so poor that it could not withstand normal handling and erection procedures.

The adhesion tests showed poor results in every case, regardless of the test location, with all detachment occurring cohesively within the zinc primer. No patterns or trends were found. This is significant when considering application and thickness issues. Even though the thickness of each coat was not within the specified tolerances, the lack of a pattern or trend indicates that the behavior of the system was the same in areas where the coatings were below, within, and above the specified thicknesses. It is clear that a fundamental problem has occurred as essentially all surfaces of all members exhibit the weak coating.

The fact that steel shot was used as the abrasive, or the quality of cleaning could not be conclusively confirmed, has no bearing on the failure as the zinc adhered well to the substrate - the failure occurred cohesively within the primer itself.

Other possible causes might include overspray or dry spray, or improper agitation of the zinc during application. While the clues do not specifically exclude these possibilities, the fact that no patterns or trends were observed tends to cast doubt that the application could be so consistently and uniformly deficient that the entire coating would fail because of it. Overspray and dry spray problems typically occur at inside corners where two or more surfaces come together, but the adhesion results in these areas were no different than the results in the middle of the web. Contamination of the zinc prior to the application of the epoxy can also be ruled out because the intermediate coat adhered well to the zinc (the break was within the zinc, not between the zinc and the epoxy).

The key clue provided in this case involves the time of year that the work was done. The northeast is very cold and dry in the winter, but the inorganic zinc requires moisture to cure. As the relative humidity is reduced, the length of curing time prior to overcoating increases. Unfortunately, this is contrary to the need to ship steel on a tight schedule. In this case, while the shop used heat to maintain adequate temperatures for applying the coatings, they did not take into account the effect that very low humidity has on the length of cure prior to overcoating, which can be as long as days. In this Challenge, the zinc primer was topcoated before it reached adequate cure due to low humidity. This is the cause of the weakened film.

One test supported by many coating manufacturers as a means for determining whether an inorganic zinc rich primer is cured sufficiently for overcoating is ASTM D4752, Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub. Another test used by some is pencil hardness in accordance with ASTM D3363, Test Method for Film Hardness by Pencil Test. Even if tests are not used, it is critical to recognize that the curing of inorganic zinc primers prior to topcoating is dependent upon time, temperature, and humidity.

The KTA laboratory can also determine the extent of curing after-the-fact by examining samples using analytical techniques such as infrared spectroscopic analysis.

Challenge #13 received an excellent response with almost 1/3 of the answers correctly determining the cause. Incorrect answers included concerns with the profile created by steel shot and contamination of the steel from oil, grease, and byproducts from the heaters used in the shop. Contamination on the surface of the zinc was also mentioned along with condensation, overspray and dry spray. Use of the wrong solvent in the primer, solvent entrapment and amine blush from the epoxy also received votes. But the greatest number of responses by far focused on the excessive thickness of the coating, especially the zinc primer.

The winner of Challenge #13, by random drawing of the correct answers is Julian Hay of Associated Coating Services, Toronto, Canada.