Technical Info Sheet #16
People often talk about “galvanizing.” However, there are several zinc application methods, each with its pros and cons. It is therefore important to understand what these differences can lead to.
See also “Thermal galvanizing – Speech confusion in galvanizing – different techniques for galvanizing” and TECHNICAL INFOBLAD 11: ZINKAPPLICATION METHODS.
HOT-DIP GALVANIZING
Hot dip galvanizing is a metallurgical process that occurs when we immerse steel in molten zinc. The coating is created by Fe-Zn diffusion at the steel surface, followed by the formation of Zn-Fe alloys that are “metallurgically anchored” to the steel surface. Excellent adhesion and wear resistance is the result.
After the workpiece 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 completely closed, fairly thick coating is formed on the surface.
ZINC SPOUTS
Zinc(Zn) – but nowadays mainly – zinc-aluminum (Zn-Al) spraying is also known as “flame galvanizing,” “schooperen” or “metallizing. In this process, Zn (or Zn-Al) wire (or powder) is passed through a spray gun and heated (flame temperature > 2500 °C) causing it to melt. The molten metal is sprayed with compressed air onto the steel surface after first blasting the surface. No diffusion takes place, as in hot-dip galvanizing, but a physical bond is established. The surface roughness of the steel is very important for good adhesion.
Because of the way in which the coating is created in zinc spraying (Zn droplets solidify on top of each other), this layer is porous to a greater or lesser extent (the porosity is more than 10%). To ensure adequate corrosion resistance, these coatings are almost always sealed with an organic coating. A paint or powder coating is perfectly suited for this purpose. In order to spray on thicker layers and to maintain sufficient mechanical strength of the coating, the smallest possible droplets should be sprayed.
APPLICATION
Zinc spraying is a good alternative to discontinuous hot-dip galvanizing provided an additional organic coating is applied. The technique is often used for large components such as bridges, passerelles, and the like. These large composite objects cannot be immersed in a hot-dip galvanizing bath because of their size.
The technique is also used for the renovation/restoration of antique fencing. Such fencing is sometimes composed of rivets, dovetails and wedges or consists of a, for that time, typical chemical composition. In either case, such fencing is unsuitable for discontinuous hot-dip galvanizing.
PROTECTION LIFE OF THE ZINC LAYER
Technical Information Sheet 10: “Corrosion resistance of hot-dip galvanized steel” states that the corrosion rate is proportional to the layer thickness. With discontinuous hot-dip galvanizing, a completely closed coating is built up in one dip with uniform layer thickness over the entire object.
With zinc spraying, you build up the coating in superimposed strips. Layer thickness can vary greatly due to the manual nature of spraying and not all surfaces (especially internal hollows) are accessible. Because the zinc coating applied by zinc spraying is usually covered with a paint system, the zinc coating will not be in direct contact with the outside atmosphere. Should no additional coating be applied, a distinction applies with respect to the protection life of the zinc coating. In certain circumstances, the corrosion resistance of a pure zinc (Zn) layer may differ from the corrosion resistance of a layer consisting of a Zn-Al alloy as applied in zinc spraying. Furthermore, it is important to check the applied Zn or Zn-Al layer for layer thickness before applying the organic coating. Many layer thickness measuring instruments only measure the total layer thickness applied. So taking measurements afterwards does not provide sufficient assurance of the applied system.
FORMATION
During hot-dip galvanizing, deformation may occur. But if you follow the recommendations in Technical Information Sheet 3: “Thermal deformation by galvanizing,” deformation will be kept to a minimum. Zinc spraying does not produce deformation of the object because the object does not need to be heated.
HOLLOW STRUCTURES
The dipping process of hot-dip galvanizing ensures that the interior walls of steel structures with hollow parts (both profiles and tubes, as well as assemblies such as vessels and pipes) are also provided with a zinc layer without any problems and completely. With zinc spraying, the interior walls of inaccessible, hollow parts cannot be zinc coated (or zinc-aluminum coated).
MAXIMUM DIMENSIONS
In hot-dip galvanizing, the zinc bath dimensions limit the dimensions of the steel structure that can be galvanized. The “double dip” technique can provide a solution. Another solution involves dividing the steel structure into parts that do fit in the bath. The parts are then assembled together. Zinc spraying allows us to repair damage to the zinc coating by welding (see Technical Information Sheet 6: Welding after galvanizing). Theoretically, there are no size limitations with zinc spraying.
HYDROGEN EMBRITTLEMENT
With hot-dip galvanizing, especially with high-strength bolts, occasional hydrogen embrittlement can occur. This is counteracted with special mordants or by blasting. The problem of hydrogen embrittlement does not exist with zinc spraying.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 2063-1
Thermal spraying – Metallic and other non-organic coatings – Zinc, aluminum and their alloys – Part 1: Planning of corrosion protection system – Considerations for parts design and quality requirements
EN ISO 2063-2
Thermal spraying – Metallic and other non-organic coatings – Zinc, aluminum and their alloys – Part 2: Conditions for performing corrosion protection by thermal spraying processes
TECHNICAL DATA SHEET 3
Thermal deformation due to galvanizing
TECHNICAL DATA SHEET 6
Welding after hot-dip galvanizing
TECHNICAL DATA SHEET 10
Corrosion resistance of hot-dip galvanized steel
TECHNICAL DATA SHEET 11
Zinc application methods
HOT-DIP GALVANIZING – CONFUSION OF TONGUES IN GALVANIZING – DIFFERENT TECHNIQUES FOR GALVANIZING
