Technical Sheet #2
This “touch-up” procedure applies to areas that are not galvanized by not reacting with the molten zinc during immersion in the zinc bath. It can also be used for hot-dip galvanized items that are damaged during storage, transportation, assembly or as a result of mechanical processing.
WHAT CAUSES UNGALVANIZED SPOTS AND DAMAGES?
It is possible that some ungalvanized areas may be visible after galvanizing. In almost all cases, this is caused by poor design or execution of the object to be galvanized. In order to galvanize, it is important that when the object is dipped in the pretreatment liquids, and then also the zinc, sufficient galvanization holes have been made. In addition, it is important that the holes be of the correct size. If this is not the case, there may be spots during immersion in the pretreatment fluids that are not adequately cleaned due to
the formation of an “air chamber.”
If the steel is insufficiently cleaned, then no chemical reaction between the steel and zinc takes place. An ungalvanized spot results. Damage, on the other hand, can occur during all movements in production, during transport, during storage and during assembly. Regardless of the cause of the ungalvanized spot, it is recommended that these spots be properly repaired.
Sometimes a conscious decision is made to leave certain surfaces ungalvanized because they will not be joined by welding until after galvanization. This is the case, for example, with fatigue-sensitive bridge structures, large frames or extra-long beams. In such situations, the areas that must remain free of zinc are treated with an anti-zinc paint prior to galvanization. During the pretreatment process, this paint protects the steel from forming a zinc layer. When immersed in the zinc bath, the paint burns, but no zinc layer forms in those areas. After welding the parts together, the weld and its immediate vicinity can be treated as indicated in this document.
what does the international galvanizing standard say?
Section 6.3 ‘Touch-up’ of the galvanizing standard EN ISO 1461 states: ‘Ungalvanized areas to be touched up by the hot-dip galvanizing may not exceed a total of 0.5% of the total area of a component. Each ungalvanized spot intended for touch-up shall not exceed 10 cm². If the ungalvanized area is larger, the item containing such portions must be re-galvanized, unless otherwise agreed between the client and hot dip galvanizing.’
HOW TO REPAIR?
With regard to ungalvanized spots, the standard specifies four repair methods, which you can also use to repair damage: zinc dust-rich paints, metallizing or zinc spraying (EN-ISO 2063), zinc flake products and
low-melting-point zinc alloy rods.
In practice, zinc dust-rich paints are used almost exclusively. These are primers whose weight consists of approximately 90% zinc dust (EN-ISO 3549). Often the manufacturer indicates on the packaging that the product is suitable for repair in accordance with EN-ISO 1461. People also use the designation Zinc compound or zinc-rich paint for such products. Spray cans (“zinc spray”) are usually less suitable because the required/wanted coating thickness is not easily achieved.
PRACTICAL APPROACH TO UNGALVANIZED SPOTS AND DAMAGES
To apply zinc-rich paint, use the following procedure:
PRACTICAL APPROACH TO WELDED PARTS
When a conscious decision is made to leave parts ungalvanized in order to weld them together later, the weld and its immediate surroundings can be metallized (also called zinc spraying or Schooperen). This is used primarily for bridges and other fatigue-prone structures where an extra-long, maintenance-free life is desired. It is essential here that the ungalvanized parts be thoroughly cleaned by blasting after welding. Immediately following this, metallizing must take place so that no oxides can form that will adversely affect the adhesion of the zinc layer. After metallizing, a zinc-rich paint can be applied if necessary. This seals any pores in the zinc layer and thus contributes to a further extension of the service life.
IMPACT ON LIFESPAN
Typically, a paint system offers a lesser durability than the thermally applied zinc coating Nevertheless, this particular (zinc-rich) paint system provides more than adequate corrosion protection for most applications. In addition to a barrier effect, these paint systems also have a cathodic property to protect the steel. After all, these are smaller spots that are touched up and the vast majority of the surface (namely more than 99.5% of the object) has the zinc layer which, in addition to the barrier effect, also provides cathodic protection to the steel. Thus, should the paint layer disappear completely, due to wear and tear for example, there is still ample protection available from the zinc layer. In practically all cases the life expectancy is not adversely affected by the repair. Metallizing can achieve a life comparable to that of the rest of the galvanized object.
TOUCHING UP A DUPLEX SYSTEM
How should you touch up an ungalvanized spot or damage if the galvanized steel is provided with an organic coating (Duplex system)? Annex C “Touch-up of ungalvanized or damaged areas” of the galvanizing standard EN-ISO 1461 describes this as follows. When the galvanizing plant has been notified that additional coating will be applied, the galvanizing plant will inform the client of the method of the repairs. The client and applicator of the wet or powder coating must ensure that the coating system to be applied is compatible with the touch-up method and materials used by the galvanizing plant.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 1
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 1: General design principles and corrosion resistance.
EN ISO 2808
Paints and varnishes – Determination of film thickness
EN ISO 2063
Thermal Spraying – Metallic and other inorganic coatings – Zinc, aluminum and their alloys
EN ISO 2178
Non-magnetic coatings on magnetic substrates – Measurement of coating thickness Determination of film thickness – Magnetic method
EN ISO 3549
Zinc dust pigments for paints – Specification and test methods.
