How Does Galvanising Work?
Hot Dip Galvanising was first developed over 50 years ago and still forms an essential element of defence against corrosion in the harshest of marine environments.
Hot Dip Galvanising is not just a surface coating.
If you scratch the surface, you don’t necessarily break through to bare metal because the zinc treatment forms a multi-layered deep-seated fusion with the steel.
Hot dip galvanising is completely different to cold galvanising.
Cold treatment is a surface coating e.g. electro-plating, paint (Galvafroid) or spray paint with a protective zinc additive.
There are three main parts to the galvanising process: surface preparation, treatment, and inspection.
Surface preparation of the steel before Galvanising:
Surface preparation is paramount to the successful application of any protective coating.
Hot dip galvanising relies on a metallurgic reaction between the molten zinc and the steel.
Zinc will not react with unclean steel.
Therefore, a successful galvanising finish is dependent on the thoroughness of the cleansing process.
Close inspection after immersion and cooling will reveal spots or patches.
Surface preparation methods:
- Degreasing = the elimination of dirt, paint, grease and oil residue by e.g. hot alkali/acid/ biological cleaning agents and shot/grit/sand blasting
- Pickling = the removal of mill scale and iron oxides (rust) by e.g. sulfuric/hydrochloric acid and shot/grit/sand blasting from the steel surface.
- Fluxing = a zinc ammonium chloride solutionexpels any remaining oxides and applies a protective layer which prevents any re-oxidisation before immersion in the molten zinc.
Galvanising Treatment Method:
Decontaminated steel is submersed in a bath of molten zinc.
The immersion is time critical.
The zinc must be at least 98% pure.
The zinc temperature is maintained at a minimum 435 degrees centigrade.
The molten zinc reacts metallurgically with iron in the steel to form a series of protective alloy layers.
The composition of the alloy gradually changes from the inner fusion to the outer surface.
Galvanising Process Technical Detail:
The protective alloy layers are not surface coatings.
The reaction between the two metals fuses the zinc into the base steel.
The galvanising treatment consists of 4 fused layers.
The base fusion is approximately 75% zinc and 25% iron, and it is harder than the steel it bonds to. The subsequent layers have progressively more zinc content and become more ductile.
The outside layers are not as hard, but they offer more resistance to impact and the environment.
The 4 layers are listed below in order from the steel (100% Fe, 159DPN) to the outer surface are technically known as:
Gamma - 75% Zn 25% Fe, 250DPN hardness
Delta - 90% Zn 10% Fe, 244DPN hardness
Zeta - 94% Zn 6% Fe, 179DPN hardness
Eta - 100% Zn 0% Fe, 70DPN hardness
DPN = Diamond Pyramid Hardness and the lower the figure, the softer it is.
The excess galvanising is removed by vibrating or centrifuging to smooth off the outer surface to a burnished finish.
Close physical scrutiny is the normal method.
There are technological electronic tests, but these are generally only of practical use for sample testing e.g. micron depth testing (galvanising thickness)
Galvanising Anchor Chain: The Challenge
Producing a consistent good quality finish with 100% coverage is problematic because of the interconnecting nature and onerous weight of the chain.
Individual link Issues
The link joints fit together naturally which can potentially create a little area or patch that doesn’t get completely decontaminated before bathing.
This can affect the thickness of coverage and/or cause bare spots or lead to impurities in the galvanising finish e.g. black patches.
The link joints present another problem when the chain is dipped into the molten zinc.
Too little zinc or too much zinc in those same little areas can potentially cause issues.
Too little zinc and the guaranteed thickness (measured in microns) may be compromised.
Too much zinc may result in the links sticking together.
This is generally eliminated by the vibrating or centrifuging process, but if any links remain stuck together after the zinc has completely cooled, the galvanising under the fused joint will be compromised when it is eventually forced to separate.
Continuous Length Challenges:
There are two main methods for galvanising anchor chain:
- Passing the chain through the molten zinc at a pre-determined regular speed. This is generally fine for relatively small chain, but it becomes increasingly difficult to achieve in the larger sizes because of the weight involved.
- Larger chain is normally lowered into the molten zinc in a bundle.
Bearing in mind the individual link issues that have already been specified, both methods present a challenge to the galvanising process in terms of achieving a 100% coverage.
Zinc is a natural, abundant resource essential to life on earth.
Both zinc and steel are 100% recyclable.
Galvanised steel has very little negative impact on the environment and requires little or no maintenance, so no additional energy or resources are used to keep it in service.
The initial detrimental impact of production is therefore mitigated by long life and recyclability.
MF Exclusive Solid Zinc Galvanising
MF have developed an extra element to their galvanising process.
They have named it Solid Zinc and refer to it as a duplex treatment where both aspects are hot dip molten zinc galvanisation.
Solid Zinc is an MF exclusive process which successfully adds extra galvanising thickness and extends the galvanic protection against corrosion by as long as 25%
Solid Zinc Facts and Benefits:
Improved outer surface Eta Layer with 25% more zinc giving better protection to the next Zeta layer
100% galvanising success despite the awkward nature of the process experienced by all manufacturers with chain links.
Smooth, brilliant zinc finish with no blemishes.
Reduced white oxidisation - this unsightly blooming effect normally occurs on galvanising as soon as the chain is exposed to the elements.
Checking the condition of your galvanised chain
Light white oxidisation commonly occurs when galvanising is first introduced to an outdoor or marine environment, although it may also appear at any stage of the working life of the chain.
A light covering is quite normal because the zinc reacts with hydrogen and oxygen when the galvanising first gets wet - this first showing normally disappears with active use in seawater.
Brown staining is the first sign of rust and the most common weathering issue with galvanised chain. Rusting will eventually happen to even the best galvanising.
Protection against corrosion/rust is down to the thickness (number of microns), the purity (absence of any impurities in the process) and the overall percentage coverage of the zinc.
N.B. Pollutants present in the sea water or physical contact with any other contaminants/chemicals can have a major effect on how long corrosion/rust takes to appear, and the rate of deterioration once it starts, rather than the problems being caused by a defect in the galvanising process.
Regular washing in clean, fresh water with a little detergent (deck wash) may help to prevent the pollution/contamination from affecting your anchor chain.
Black patches aren’t very common, and they are aesthetically unseemly. However, they are generally the result of minor impurities in the zinc, and not corrosion.
Keep a look out for the first signs of rust. Don't let it progress as far as the picture above left. Take action as early as possible to protect the link with a cold galvanising treatment (spray)
Rust marks like those in the picture above right may be caused by impurities migrating from an outside source. Regardless, any sign of rust needs to be investigated and treated as soon as possible to avoid deterioration.
Lofrans New Galvanising Advisory
Occasionally new chain will have little marks which look like the galvanising is missing.
However, hot dip galvanising is not a coating but a chemical fusion (as explained above), so these flaws are most likely caused by a tiny bit of impurity contamination in the process.
Lofrans attest that these blemishes will not have a detrimental affect on the longevity or effectiveness of the galvanising on their chain.
The picture below shows a single mark on this Lofrans galvanising: