The corrosion resistant life of a zinc coating is the same, regardless of the process by which it is applied

True or False?

Zinc coatings are applied by a variety of methods. These include; Thermal metal spraying, electrolytic deposition (zinc electroplating), diffusion processes, mechanical plating (peen plating), zinc rich paints and hot dip galvanizing.

During the past two or three decades, there has been a trend towards the grouping of all these processes under the common term galvanizing. To illustrate, in recently published “Universal Dictionary”, the term galvanizing is defined as “to coat iron or steel with rust resistant zinc, by spraying, immersion or electrolytic deposition”. Thus we now have terms such as cold galvanizing, electro galvanizing and mechanical galvanizing.

It is unfortunate that this irreversible trend has lead to the misconception that all these zinc coatings are interchangeable and that they provide an equal degree of corrosion protection. This is of course far from the truth.

For effective long-term corrosion prevention, the following properties of a coating must considered:

Coating thickness

Zinc is a wasting protector i.e. the coating is gradually reduced in thickness usually in a uniform manner, over a period of time. The rate at which coating thickness is reduced, is determined by the corrosivity levels pertaining in a given environmental but the thicker the coating, the longer the life. It follows that the thin zinc electroplated coatings will not endure in corrosive environments to the same degree as substantially thicker hot dip galvanized coatings. To a lesser degree, the same applies to continuously hot dip galvanized sheet where a variety of coating thickness is available. The thicker sheet coatings should be specified for corrosive applications.

Adhesion

For durable barrier protection, effective coating adhesion is of paramount importance. In the case of hot dip galvanizing and zinc diffusion coatings, this is achieved by metallurgical reaction between zinc and iron. A series of iron/zinc alloys are produced which effectively bond the coating to the steel substrate.

In other zinc coatings such as zinc metal spray and the range of zinc rich paints, alloys are absent while adhesion is largely influenced by the extent to which effective steel surface preparation has been carried out.

Permeability

Long term barrier protection is achieved not only by way of the corrosion resistant properties of the coating material, but also by the extent to which the coating is capable of preventing permeation of moisture and corrosion inducing substances through to the underlying steel. Some zinc based coatings are porous to a lesser or greater degree. This problem is rarely encountered with zinc coatings applied by the hot dip galvanizing process.

As with any other coating, either organic or inorganic, selection of the most suitable zinc coating is largely influenced by the intended application which is being considered. It is once again a question of horses for courses.

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