Zinc melts at 419°C. The temperature of the zinc bath must remain above this in part because the bath, by immersing much cooler objects, will cool down. Also, when the object is hoisted out, excess zinc will have to run off the profile surface. Likewise, one will need to allow sufficient time to make repairs in the event of an emergency. For example, if the energy supply to the heated bath should stagnate, solidification of the zinc in the bath must be prevented. Solidification would shrink the volume of the zinc, creating a gap between the zinc bath and the solidified zinc. This would make it impossible to melt the zinc after solidification without putting thermal stress on the pan.

Too high a temperature, apart from a high energy bill, also causes damage to the zinc pan. This steel pan is accelerated to dissolve at temperatures above 480°C, as zinc reacts particularly aggressively with iron from this temperature. In addition to optimum results in galvanizing, these are the reasons why the zinc bath is heated to a temperature of 450°C.

There are also higher temperature plants and ceramic zinc baths for special applications, but these are not included in this Technical Information Sheet.
The galvanized coating is created by an Fe-Zn diffusion process at the steel surface, followed by the formation of Zn-Fe alloys that are “metallurgically anchored” to the steel surface. This provides excellent adhesion and wear resistance. The moment the object is immersed in the zinc bath and the chemical reaction starts, first the Zeta layer is formed. Further diffusion then creates the Delta layer and Gamma(-double) layer (formerly called the gamma double layer because of its structure). Upon hoisting out of the zinc bath, the liquid zinc runs off the object and, upon solidification, forms an almost pure zinc layer (Eta layer = at least 98% pure zinc) which is also called solidification layer for understandable reasons.

The standard for galvanizing (EN-ISO 1461) cites aspect of adhesion and immediately mentions that there is no known suitable and standardized method for testing the adhesion of the galvanized coating.
It is clear from the foregoing explanation that the adhesion of the zinc coating usually does not need to be assessed. The chemical anchoring of the zinc-iron alloy layers is more than sufficient for almost all applications.
For special applications, however, a client may have an interest in a certain minimum adhesion of the zinc coating. Consider, for example, the running surfaces of crane runways. In such cases, test methods can be agreed in advance
to assess adhesion.

Thus, the dolly test (EN-ISO 4624), normally applied for organic coatings, can be used, or the “knife test” in which a sharp and sturdy knife (similar to an oyster knife) is used. Sometimes reference is also made to the diamond test (ISO 16276-2 or EN-ISO 2409), as is also the case for organic coatings. The hammer test, according to the American standard ASTM A123, is hardly used nowadays. At most, this test shows that the zinc coating is harder than the underlying steel. Dropping a hammer on the “hard” zinc layer, which will therefore not deform, will cause the zinc to separate from the steel because the steel is “tougher” and does dent due to the impact of the hammer.

In rare cases, there is no or virtually no adhesion between zinc coating and the steel. In these cases, only a solidification layer of excess zinc is present. With the slightest mechanical stress, such as a tap, fall or bending, the entire zinc layer comes off. This is caused by the surface condition and chemical composition of the steel. Some steels, such as stainless steel*, manganese steel, self-hardening steel, spring steel and auto-steel, are simply not suitable for galvanizing. However, this never occurs with regular structural steels. So this has nothing to do with adhesion, but with the chemical process taking place insufficiently, which disturbs the formation of zinc-iron alloy layers. This is immediately visible after galvanizing, which makes galvanizing a reliable system. Any abnormalities become visible immediately, unlike, for example, paint or powder coatings, where problems sometimes only appear after a period of time.

*Some types of stainless steel can be provided with a galvanized coating, however, which can be determined by practical testing.