FAQ

FREQUENTLY ASKED QUESTIONS

Questions

Question 1 :
We have a large balcony project and the Architect has requested that it be painted after Hot Dip Galvanising. We have heard that most paint will not adhere directly to freshly galvanised steel. Is this true and how should we overcome this problem?
Answer >>

Question 2 :
How long does it take to galvanise steel?
Answer >>

Question 3 :
Can I specify a thicker coating than normal and if so, will I achieve a correspondingly increased lifespan from my steel?
Answer >>

Question 4:
We recently had to carry out some remedial work on site, prior to erecting a Hot Dip Galvanised steel structure. The architect on the job was concerned that we might have intrinsically damaged the corrosion protection of the steel. Is this the case ?
Answer >>

Answers

Answer 1:
We have a large balcony project and the Architect has requested that it be painted after Hot Dip Galvanising. We have heard that most paint will not adhere directly to freshly galvanised steel. Is this true and how should we overcome this problem?

In general most paints will not adhere to freshly galvanised steelwork. The traditional method was to etch the surface with etch primers, T-wash or by a light sweep blast. All of these are expensive and in the case of some primers can be unpleasat to work with and cause environmental issues.

Thankfully this is no longer necessary. Ongoing research and development has led to the introduction of Galvacoat, a top quality paint that will adhere directly to freshly galvanised steel. This two pack polyurethane paint is readily available in all RAL and BS colours and is simple to apply by spray or brush.

Galvacoat has been approved by Dublin Corporation and specified by many top practices. There are several high profile applications where Galvacoat has been used details of which can be found by clicking on the ‘Case Histories ’ section of this website or on the links below.

Answer 2
How long does it take to galvanise steel?

Leadtimes can vary depending on the time of year and how much demand there is within the industry, however most orders are completed within one week. The actual process of Hot Dip Galvanising takes approximately half a day from start to finish so small urgent items are often processed within 24 - 48 hours.

In order to plan effectively we operate a Booking System. This allows customers the opportunity to pre-book their steel and agree delivery schedules. This eliminates any uncertainty and means that even the most demanding leadtimes can be met. In one recent project we were required to process 600 tonnes in ten weeks. We were able to programme three loads of 20 tonnes per week for the duration and ensured that the project ran smoothly from start to finish.

We would ask that you take care to contact us at an early stage in the planning process so that we can advise you on suitable methods of design and fabrication. Many delays are caused by inappropriate design and a lack of understanding of the galvanising process ...Bath Capacity

Answer 3
Can I specify a thicker coating than normal and if so, will I achieve a correspondingly increased lifespan from my steel?

Current indications are that the normal coating thickness of 85 microns will protect most articles for well in excess of 40 years. Therefore, increasing the coating thickness is not necessary except in extreme circumstances. However, should it be required there are two ways it can be achieved:

  • by surface roughening;
    Grit blast the surface to Sa2.5 using chilled angular iron grit of size G24. This roughens and increases the surface area of steel in contact with molten zinc and leads to an increase in the weight per unit area of a hot dip galvanised coating by up to 50%.
  • by using reactive steels. Both of these methods will increase the zinc pick-up and extend the life to first maintenance accordingly.
    A thicker zinc coating can be achieved if the article to be galvanised is manufactured from a reactive steel.
    The constituents in steel that have the greatest influence on the iron/zinc reaction are silicon and phosphorous. When an article is removed from the galvanising bath, a pure zinc layer adheres to the alloy layer. The reaction rate in these steels can be so high that this zinc layer is transformed completely to zinc-iron alloy before the article has time to cool.
    The result is a coating of equal or increased thickness which can be much duller in appearance than normal. We recommend that you consult with us prior to specifying an increased coating to confirm that the desired result can be achieved. It also allows us to monitor the steel before and during the galvanising to ensure compliance.

Answers 4

We recently had to carry out some remedial work on site, prior to erecting a Hot Dip Galvanised steel structure. The architect on the job was concerned that we might have intrinsically damaged the corrosion protection of the steel. Is this the case?
Small areas of galvanising may be damaged by operations suchas cutting or welding. Also, small areas of damage may occur in transport and erection.

Because of the sacrificial action of zinc, localised flaws (up to 5mm in diameter) tend to be self healing and have little effect on the life of thecoating.

Nevertheless, it is often aesthetically desirable to renew the coating over small areas as well as larger areas using one of the following techniques.

  • Thoroughly wire-brush the affected areas and apply several coats of zinc-rich primer (min. 90% zinc content) to give a coating thickness at least equivalent to the original galvanising.
    Many of these paints deposit zinc at about 150g/m/coat; This will usually give a dull coating as illustrated. A top coat of of 'bright galvanising spray' may be then applied to achieve a suitable colour match Areas of galvanising which are damaged - for instance by welding - may be readily made good.
  • Thoroughly wire-brush then heat the bare area with a blow torch to 300°C and apply special zinc alloy rod;
  • Grit-blast the affected area to Sa3 and zinc spray. A 100µm sprayed coating confers corrosion protection equivalent to an 85µm galvanised coating. Painting with zinc-rich paint is usually the simplest method but if a colour match is important, then zinc alloy rods are more suitable.

IS EN/ISO 1461 gives guidance on all matters such as coating thicknessand renovation procedures as well as size of areas acceptable for treatment. Areas of galvanising which are damaged - for instance by welding - may be readily made good.

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Unit 5, Industrial Estate, Magherafelt Road, Draperstown, Northern Ireland, BT45 7JT, Tel:+ 44 28 796 27657, email: draperstown@sperringalvanisers.com
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