Technical Info Sheet #19
The terms galvanizing or galvanizing refer to a range of different methods of protecting steel using zinc coatings. Hot-dip galvanizing protects steel from corrosion, possibly in combination with a paint system (duplex system). Steel is also sometimes preserved with a paint system alone. This information sheet compares the properties of hot-dip galvanizing and paint systems.
See also “Hot-dip galvanizing – Speech confusion in galvanizing – Different techniques for galvanizing” and TECHNICAL INFOBLAD 11: ZINKAPPLICATION METHODS.
PACKAGING AND TRANSPORTATION TREATMENTS
After the object to be galvanized is manufactured in a construction workshop, it is delivered to a hot-dip galvanizing plant where the entire object is dipped into a 450°C zinc bath. A metallurgical reaction takes place and a completely sealed, fairly thick, coating is formed on the surface. Virtually all steel products can be galvanized this way including construction profiles.
Discontinuous hot-dip galvanized objects are used almost exclusively in an outdoor environment, although architects are increasingly choosing to use them in indoor areas as well for aesthetic reasons. After all, galvanized steel gives an industrial look, a tremendously long maintenance-free life and is 100% circular.
UPDATING ON SITE
This is rarely necessary for hot-dip galvanized steel. Unless changes have to be made on the construction site (such as drilling, sawing and grinding) that damage the zinc coating. By observing the
repair procedure (see also Technical Information Sheet 2: Procedure for touch-up), a service life as calculated is also obtained for the repaired areas.
In the event of (minor) damage to a paint system, a very careful inspection and ditto touch-up is required. After all, the short-term consequences for corrosion formation can be serious. This is because paint systems lack cathodic protection (see Technical Data Sheet 20: Cathodic protection and the effect of sharp edges).
COMPLETE PROTECTION
In hot-dip galvanizing, the steel is completely immersed in molten zinc. The result is a completely sealed and ubiquitous layer of zinc. Even on the inner walls of hollow parts
(such as pipes, tubes and vessels). Whether a paint system is applied in the workshop or on the construction site, the inner walls of pipes and the (possibly) hard-to-reach places remain unprotected.
Of course, it is precisely those hard-to-reach places that are a source of rapid corrosion.
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 protected by a paint system and which must be applied entirely
or partially 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 fluctuating temperatures are no problem. The properties of paint systems deteriorate
rapidly at operating temperatures above 90°C. Paint systems also have poor resistance to 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 minor damage 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 usually for well over 50 years. A paint system acts only as a barrier. With the difference that this barrier is of lower quality and easily damaged (mechanical damage, UV radiation, et cetera). Small damages such as scratches and cracks lead almost immediately to rust formation. And this rusting spreads further under the paint layer: under-rust. Furthermore, one must regularly clean a paint layer and also repair or completely repaint it in between.
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 applicator. Even in hard-to-reach areas and on edges and corners (where a paint system is vulnerable anyway), the paint layer thickness is smaller than elsewhere.
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 substrate and the zinc coating. In paint systems, the adhesion is usually about 10 times smaller. Therefore, there is only mechanical adhesion. The paint “sticks” to the steel, so to speak.
HARDNESS/WEAR RESISTANCE
In hot-dip galvanizing, the hardness of the Fe-Zn 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, corrosion protection is guaranteed. Consider scaffolding tubes as an 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 is reducing the concentration of substances in the atmosphere that can affect the zinc coating. This further extends the life of hot-dip galvanized steel.
Paint systems are affected by the sun (UV radiation), temperature fluctuations and wind. This requires regular maintenance in the form of cleaning and doing repairs. After the first 15 to 20 years of use, complete repainting of the system is even necessary.
Assuming a calculated life span of 50 years, the total purchase and operating costs of the paint system will be many times higher than in the case of hot-dip galvanizing.
Only in cases where the galvanized steel is placed in an environment where the conditions are such that the zinc layer corrodes quickly will a wet-paint system be preferable. Examples include installation at sea (drilling rigs, coasters, buoys) or directly on the coastline near ports. Especially in those cases where there is a combination of seawater in a (sub) tropical climate. Also in certain indoor applications with certain sulfurous gases and continuous humidification, it is often better to choose an appropriate wet paint system.
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
TECHNICAL DATA SHEET 2
Procedure for updating
TECHNICAL DATA SHEET 9
Inspection of discontinuous hot-dip galvanized steel
TECHNICAL DATA SHEET 12
The mechanical properties of hot-dip galvanized steel
TECHNICAL DATA SHEET 20
Cathodic protection and the effect of sharp edges
GUIDE TO PROTECTING STEEL FROM CORROSION
HOT-DIP GALVANIZING – CONFUSION OF TONGUES IN GALVANIZING – DIFFERENT TECHNIQUES FOR GALVANIZING
