Technical Info Sheet #17
The terms “galvanizing” or “Galvanizing” refer to a range of different methods of protecting steel using of zinc coatings.
See also “Hot-dip galvanizing – Confusion around hot-dip galvanizing – Different techniques for galvanizing” and TECHNICAL INFOBLAD 11: ZINKAPPLICATION METHODS.
WHAT IS ELECTROGALVANIZING?
Electrogalvanizing is based on the principle of “electrodeposition” (the deposition of substances under the influence of electricity). After chemical pretreatment (degreasing, pickling), the steel objects are switched as a cathode in an electrolyte consisting of an aqueous solution of a zinc salt (such as ZnCl2). The anode consists of zinc. Under the influence of direct current, the Zn2+ ions migrate to the cathode, where they precipitate as metallic zinc. As with hot-dip galvanizing, the application for electrogalvanizing is both continuous (steel strip, wire, pipe) and discontinuous (by batch). The discontinuous process is done both in hanging baths (usually no longer than 3 meters) and in perforated, rotating drums (small mass parts such as bolts, nuts and brackets).
DISCONTINUOUS HOT-DIP GALVANIZING
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 in 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 in 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 spaces as well for aesthetic reasons.
After all, galvanized steel gives an industrial look, a tremendously long maintenance-free life and is 100% circular.
ZINC COATING THICKNESSES
Electrogalvanizing
Discontinuous hot-dip galvanizing
See Tables 1 and 2 of Technical Info Sheet 9
Rule of thumb: the minimum coating thicknesses for discontinuous hot-dip galvanizing are at least 2 to 5 times greater than the maximum coating thicknesses for batch electrolytic and continuous electrolytic galvanizing.
POST-TREATMENTS
Apart from the application of a paint or powder coating system that applies to both galvanizing processes, electrogalvanized steel products (unlike discontinuous hot-dip galvanizing) are rarely used without post-treatment.
To maintain their smooth and shiny appearance and their decorative effect and/or to improve their low corrosion resistance (for example, avoidance of white rust), there are various post-treatments that we can apply to electrogalvanized steel products.
In order to obtain a shiny appearance, gloss enhancers are added to the electrolyte.
Electrogalvanized products are usually also “passivated”
by means of conversion coatings. Depending on the nature and thickness of this passivation layer, the coatings are given one of the following colors: blue, yellow, green and even black.
HYDROGEN EMBRITTLEMENT
In both discontinuous hot-dip galvanizing and batch electrogalvanizing, the steel parts are pickled during pretreatment. In addition, hydrogen generation occurs during electrogalvanizing. This can cause hydrogen embrittlement. For fasteners, this can lead to breakage. With discontinuous galvanizing,
the chance of hydrogen embrittlement is many times less.
APPLICATIONS
Discontinuous hot-dip galvanized and electrogalvanized items have totally different properties in terms of corrosion resistance. Consequently, they address completely different applications. Electrogalvanized fasteners are unsuitable for creating joints between discontinuously hot-dip galvanized objects in outdoor applications due to their limited coating thickness. In air-conditioned indoor areas, electrogalvanized materials may be chosen, however, not in the case of applications in humid areas and where condensation may occur (such as cold stores).
STRENGTHS AND WEAKNESSES OF ELECTROGALVANIZING
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN 10240
Internal and/or external protective coatings for steel pipes – Specifications for hot dip galvanized coatings applied in automated plants
EN ISO 10684
Surface treatment of threaded fasteners – hot dip galvanizing
EN ISO 2081
Metallic and other coatings – Electrolytically applied coatings of zinc with additional treatment of iron or steel
EN ISO 10152
Electrogalvanized cold-rolled flat steel products – Technical conditions of delivery
TECHNICAL DATA SHEET 3
Thermal deformation due to galvanizing
TECHNICAL DATA SHEET 10
Duration anti-corrosion effect of a hot-dip galvanizing layer in the atmosphere
TECHNICAL DATA SHEET 11
Zinc application methods
‘Thermal galvanizing – Speech confusion in galvanizing – Different techniques for galvanizing’
