Technical Info Sheet #29
The terms ‘Galvanizing’ or ‘Galvanizing’ refer to a range of different methods of protecting steel using zinc coatings. See also ‘Hot-dip galvanizing – Speech confusion in galvanizing – Different techniques for galvanizing’ and Technical Infoblad 11: Zinc application methods. We would also like to refer to Technical Info Sheet 19; Discontinuous hot-dip galvanizing vs. paint systems
Hot-dip galvanizing protects steel from corrosion, possibly in combination with a paint system (duplex system). It is sometimes claimed that cold zinc, being a paint system enriched with zinc oxides, gives the same result as hot-dip galvanizing. It should be noted that the name cold zinc creates expectations that cannot be met. It is suggested that the only difference between the two systems is that one must be applied at a high temperature and the other at a “normal” ambient temperature. Sometimes they even call it “electroplating system.” So it has nothing to do with that.
So cold zinc is nothing but a paint. Its characteristic difference from other paint systems is that a large amount of zinc powder is introduced into the binder of the paint. This allows a degree of cathodic protection in addition to barrier action by the paint. This Info Sheet compares the properties of hot-dip galvanizing and those of cold zinc.
PACKAGING AND TRANSPORTATION TREATMENTS
Hot-dip galvanized steel has excellent mechanical properties that make the galvanization layer abrasion and impact resistant (see also Technical Information Sheet 12: The Mechanical Properties of Hot-Dip Galvanized Steel). This makes hot-dip galvanized steel resistant to the (often) rather rough handling during loading, unloading and transport. Because of this good property, hot-dip galvanized parts are bundled together without the use of packaging materials.
Packing and transporting steel parts or steel assemblies that have been coated with paint involve material and labor costs that should not be underestimated. There must be absolutely no mutual contact between the painted parts to avoid damage to the paint layer. This is why wooden, cardboard or paper spacers are used. Therefore, less material can also be transported per freight.
UPDATING ON SITE
Touch-ups or repairs are rarely necessary for hot-dip galvanized steel. Unless changes have to be made on site (such as drilling, sawing and grinding) that damage the zinc coating. By observing the repair procedure (see also Technical Data Sheet 2: Procedure for touch-up), it is possible to protect these repaired areas with zinc-rich paint. Colloquially and on packaging, this is referred to as cold zinc.
Good inspection is also required during and immediately after installation for objects conserved with cold zinc. The paint layer must be in good condition to form the best possible barrier against influences originating from the atmosphere. Indeed, the short-term consequences for corrosion formation can be severe. This is because while cathodic protection is present with cold zinc, it is significantly less than in the case of a galvanized coating obtained by hot-dip galvanizing (see Technical Information Sheet 20: Cathodic Protection and the Effect of Sharp Edges).
COMPLETE PROTECTION
In hot-dip galvanizing, the entire steel object is immersed in molten zinc. The result is a completely closed and therefore ubiquitous layer of zinc. So also on the inner walls of hollow parts (such as pipes, tubes and vessels). Whether a paint system such as cold zinc is applied in the workshop or on the building site, the inner walls of pipes and the (possibly) hard-to-reach places remain unprotected. Of course, it is precisely these poorly accessible areas that are a source of rapid rusting because dirt and moisture often accumulate there as well.
In addition, with discontinuous hot-dip galvanizing, it is not necessary to round off the edges and sides. Due to the fact that the galvanized coating results from a chemical process, a sufficiently thick zinc layer is formed even on a sharp edge. When using cold zinc, it is definitely necessary to bevel (1mm) or round off (r=2 mm) these edges. Since paint is a liquid, it will recede from an edge during the drying time because of surface tension.
INFLUENCE OF WEATHER CONDITIONS
Hot-dip galvanizing can be done year-round, transportation and assembly takes place in all conceivable weather conditions. A steel project whose parts are to be protected by a paint system and all or part of which must be applied on the construction site often incurs serious delays. This is because it is necessary to wait for the appropriate weather conditions in which to apply the paint. Temperature, humidity and wind are very important parameters here
TEMPERATURE RESISTANCE
Hot-dip galvanized steel can almost always be used without problems at temperatures from -50°C to +200°C. Rapidly changing temperatures are not a problem in this regard. The properties of paint systems deteriorate rapidly at operating temperatures above 90°C. Paint systems are also poor at withstanding large temperature fluctuations.
CORROSION PROTECTION
The hot-dip galvanizing layer on steel is not only a barrier to “corrosive elements” in the environment; the galvanizing layer also provides “cathodic protection. This prevents small damages from spreading. There is no “under rust. All in all, hot-dip galvanizing provides rust- and maintenance-free protection for steel in most environments, and typically for well over 50 years.
A paint system such as cold zinc acts primarily as a barrier. With the difference that this barrier is of inferior quality and is easily damaged (mechanical damage, UV radiation, etc.). Minor damage such as scratches and cracks lead to rust almost immediately with normal paint systems. And that rusting continues to expand under the paint layer into under rust. Furthermore, one must regularly clean a paint layer and also repair or completely repaint in between. The advantage of cold zinc is that the paint contains a relatively large amount (proportion by weight of about 90%) of zinc oxides and therefore provides some form of cathodic protection to the steel, thereby delaying rusting and also reducing under-rust.
DECLOSURE
Hot-dip galvanizing provides a completely closed and uniform coating whose minimum thickness is guaranteed by standards (see Technical Data Sheet 9: Inspection of discontinuous hot-dip galvanized steel).
The film thickness of paint systems depends not only on the skill of the personnel. Even in hard-to-reach areas and on edges and corners (where a paint system is vulnerable anyway), paint layer thickness is less than elsewhere on the object.
HECHTING
Because of the way a zinc coating is created (Fe-Zn diffusion), the adhesion of the zinc coating is metallurgical/chemical in nature. This creates an inextricable link between the steel substrate and the zinc coating (see Technical Information Sheet 26: Assurance of adhesion of the zinc coating). With paint systems, the adhesion is usually about 10 times smaller, there is therefore only mechanical adhesion. The paint “sticks” to the steel, so to speak. Adhesion is crucial in a paint system and proper pre-treatment is therefore necessary.
HARDNESS/WEAR RESISTANCE
In hot-dip galvanizing, the hardness of the Fe-Zn alloy coating is greater than that of the applied steel. The galvanized coating is therefore very durable (scratch and impact resistant). Despite heavy-handed handling of hot-dip galvanized steel, the corrosion protection is still guaranteed, consider scaffolding tubes, for example.
Paint coatings are much softer than the steel and therefore less scratch and impact resistant. Consequently, a lot of maintenance and repair work is required to maintain the corrosion protection over its useful life. In short, paint coatings are much less “hufterproof” than hot-dip galvanized coatings.
LIFE
Hot-dip galvanized steel has a life of well over 50 years to, depending on the zinc coating thickness, well over 100 years when exposed atmospherically. Increasingly stringent environmental legislation has reduced the concentration of substances harmful to zinc in the atmosphere. This further extends the life of hot-dip galvanized steel.
Paint systems such as cold zinc are affected by the sun (UV radiation), temperature fluctuations and wind. This requires periodic maintenance in the form of cleaning and doing repairs. Nevertheless, the paint layer degenerates and becomes porous. After the paint layer loses its proper function as a result, even complete repainting of the system is necessary. Assuming a calculated service life of 50 years, the total acquisition and operating costs of cold zinc application will be many times higher than those of hot-dip galvanizing.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 – Part 2
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 2: Hot dip galvanizing
EN ISO 12944 part 5
Paints and varnishes – Protection of steel structures against corrosion by paint systems – Part 5: Protective paint systems
EN ISO 3549
Zinc dust pigments for paints – Specifications and test methods.
TECHNICAL DATA SHEET 2
Procedure for updating
TECHNICAL DATA SHEET 11
Zinc application methods
TECHNICAL DATA SHEET 12
Mechanical properties of hot-dip galvanized steel
TECHNICAL DATA SHEET 19
Discontinuous hot-dip galvanizing vs. paint systems
TECHNICAL DATA SHEET 20
Cathodic protection and the effect of sharp edges
TECHNICAL DATA SHEET 26
Certainty about adhesion of the zinc coating
GUIDE TO PROTECTING STEEL FROM CORROSION
HOT-DIP GALVANIZING – CONFUSION OF TONGUES IN GALVANIZING – DIFFERENT TECHNIQUES FOR GALVANIZING
