If I require a thicker, and hence, more durable galvanized coating I must specify “double dipping” or, “double spelter”

True or False?

In the context that they are used, these two terms have no relevance to general hot dip galvanizing. Double spelter is a term that has been used in the past to specify a thicker zinc coating on continuously galvanized sheet, while double dipping is the term used to indicate that a structure is too large to be immersed in its entirety in the molten zinc, in a single dip.

In the case of pre-galvanized sheet, available coating thicknesses are detailed in the relevant specification ISO 4998 (SANS 4998) and ISO 3575 (SANS 3575). To illustrate; if Z275 galvanizing is specified, the coating thickness will average about 20µM. If Z600 is called for, the average thickness will be about 42µM.

For the hot dip galvanizing of components other than sheet and wire, ISO 1461 (SANS 121) specifies various minimum coating thicknesses (not averages) that apply to articles grouped together in specific categories. If the thickest achievable coating is required, this can be secured by using the term “heavy duty coating”. To illustrate: the minimum coating thickness for steel 5mm thick and over is 85µM, where the minimum thickness if a heavy duty coating is specified, will be 105µM.

The hot dip galvanized coating is formed by a metallurgical reaction between iron and molten zinc. The ultimate coating thickness achieved is dependent mainly on steel composition, zinc temperature and immersion time with steel composition (silicon and phosphorus levels) playing the major role.

Extending immersion time during a single dip will normally increase overall coating thickness while to dip then withdraw and re-dip for a second time, constitutes a waste of production time.

Paint coatings can consist of several layers of materials that may vary generically. A hot dip galvanized coating consists of a series of Fe / Zn alloys and pure zinc, thus to withdraw and then re-immerse it is an exercise in futility.

The term double dipping featured prominently in specifications drawn up by the PWD about half a century ago. Believe it or not, it still appears in some tender documents in this so-called enlightened era.

During a technical presentation by the eminent Dutch corrosion expert, J.H.F. van Eijnsbergen, some thirty years ago (in a South African city that shall remain nameless), a delegate requested information regarding the validity of the term double dipping. When told that the term was meaningless, the delegate responded “how come the local galvanizer charges double the price if we specify double dipping?” – result, a local galvanizer (not a member of the Association) with egg on his face.

Incidentally, double dipping or double end dipping, which is the correct terminology, can be effectively done by an expert galvanizer without detracting from the aesthetic appearance of the coating.

Warning: The life of a galvanized coating is generally determined by its thickness i.e. the thicker the coating the longer the life.

Steel which contains levels of silicon and phosphorus which render it excessively reactive in the presence of molten zinc can result in the formation of an undesirably brittle coating even with a short immersion cycle. Coatings in excess of 200µM in thickness normally display brittle tendencies. For this reason, it is recommended that the minimum thickness specified for heavy-duty coatings in ISO 1461 should be regarded as the practical upper limit.

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