When a steel surface is galvanised, the zinc coating is metallurgically bonded to the original steel substrate.
As depicted in the photomicrograph (below) showing a cross-section of the edge of a galvanised article, the galvanising process naturally produces coatings at the corners and edges, which are at least as thick as the coating on the rest of the article. This is due to the reaction between iron and zinc being a diffusion reaction and thus the crystalline structure of the coating forms perpendicular to the steel surface.
As coating damage is most likely to occur at the edges, this is where added protection is needed most.
Any naturally ‘sharp’ steel edge will have a degree of radius after galvanising. The extent of this rounding is not specified in AS/NZS 4680 as it depends on steel thickness, chemistry and consequent coating thickness.
The recommendations in AS/NZS 2312:2002 Figure 3.1 relating to the painting of bare steel surfaces is shown below. This indicates that brush- or spray-applied coatings have a natural tendency to thin at corners and edges as shown in the left most part of the figure. As a result, there is a likelihood of early corrosion in these areas. This is the reason why the Standard recommends fabricators chamfer or round sharp edges.
Section 6.2 of AS/NZS 2312 provides recommendations as follows for paint coatings at edges: For edges, crevices, ponded areas and microclimates, additional protective steps (such as stripe coating) may be necessary to achieve the predicted performance. This is particularly important in aggressive environments and for critical structures where future maintenance will be difficult.
Section 10.5 of AS/NZS 2312 provides recommendations as follows for welded sections that are to be painted: In areas subject to severe corrosion, rough weld beads should be ground flush before coating; where this is impractical they should be stripe-coated using a brush. For welded sections that are to be galvanised it is important to provide a high quality weld, including removal of weld spatter prior to galvanising. This will reduce areas of potential crevice corrosion and provide a clean surface for the galvaniser to work with. Appendix C5 of AS/NZS 4680 states: It is important that the supplier remove all welding slags and spatter prior to delivery to the galvaniser.
As part of the preparation for painting it is important to remove or smooth out sharp edges, dags, weld spatter and laminations formed in the galvanised coating so that these imperfections in the galvanised surface shall not thwart the even build-up of the subsequent paint system.
It is critical that the galvaniser is advised when a hot dip galvanised article is going to be subsequently painted. The responsibility for the removal of any imperfections formed during galvanising must be agreed prior to hot dip galvanising.
AS/NZS 4680, Appendix I recommends the following practice for surface preparation of the coated surface prior to painting:
It is very important that this procedure be performed carefully to ensure that no more than 10 μm of zinc is removed. Organic paint coatings should be applied as soon as possible after abrasive blasting.
Hot dip galvanised coatings will provide a naturally rounded edge with a slightly thicker coating for structures that are to be painted. The GAA recommends that a fabricator does not normally need to round sharp edges or corners prior to galvanising, or after galvanising and prior to painting. However, the GAA do recommend a stripe coat as per the recommendations from the notes accompanying Table C1 of AS/NZS 2312 if the environment is aggressive or the product will be difficult to access for future maintenance purposes.
The use of ‘stripe’ coatings applied by brush to edges, welds, seams, etc. before each coat application is a recognized sound practice. The GAA does recommend that welds be smoothed consistent with good welding practice and that weld splatter must be removed prior to galvanising to ensure coating thickness and adhesion is to the users expectations. By following this practice, a proper surface for painting will be created.
AS/NZS 2312 provides guidance for specifications for painted surfaces. It does not provide an explicit specification for painting over hot dip galvanising.
Material supplied by the GAA.