| There are many finishes or coating
applied to fasteners; some corrosion protective, some
decorative, or there may be no added coating at all.
Specifications for fastener coatings are contained in a number
of Australian standards. |
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| (Black - Self Colour) |
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An 'as produced' finish on
carbon steel products having an oil residue which provides
some shelf life but no real corrosion protection when in
use.
Today, less than 20% of carbon steel fasteners would be
purchased plain finish. |
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Stainless steel, brass and other
non-ferrous materials protect themselves through a reaction
of the surface to oxygen, creating a protective chromium
oxide film. |
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| Zinc Plated |
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The most economic and common
fastener finish, comprising a thin coating of zinc applied
either by electroplating or mechanically. A shiny silver
grey appearance, it will normally be enhanced by a chemical
chromate passivation conversion which applies a harder
surface film. This can be clear (bluish tinge), or
iridescent yellow which is thicker and gives marginally
better protection. |
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Clear is referred to as zinc,
zinc clear, blue zinc. |
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Yellow is referred to as zinc
plate gold (ZPG), zinc yellow chromate (ZYC), zinc
di-chromate, zinc yellow pass. |
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| Cadmium Plated |
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Formerly a popular electroplated
or mechanically applied finish, looking like but giving
slightly better protection than zinc and providing increased
lubricity; also chromate converted. Very seldom used today
due to its toxicity and environmental non-acceptability. If
specified, it is usually through habit, error or ignorance
and possible confusion with zinc. |
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| Galvanised |
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A very heavy coating of zinc
applied by hot dipping in a bath of molten zinc, then
centrifuge spinning for even distribution and removal of the
excess, or mechanically cold welding a zinc powder in a
barrel rumbling process. The hot dip finish is rougher and
duller than electroplated finishes but because of the
thickness achieved, gives considerably enhanced protection.
Often it is wax coated to provide assembly lubrication. |
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| Phosphate |
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A thin, dull grey phosphate
coating obtained by insertion in a solution containing
phosphoric acid. Gives a lower level of protection than zinc
in mild environments, but gives an excellent base for
painting or organic lubrication. Often used in automotive
industry. |
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| Electro Base |
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A brass finish applied by
electroplating. Appears similar to brass and is used in
furniture or architectural fittings. |
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| Black Japan |
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A black enamel dipped finish,
used in black fittings or furniture. |
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| Black Zine |
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An electroplated zinc flash and
black chromate dip - used in dark finish appliances. |
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| Light Bronze Antique |
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Copper electroplated and dipped,
medium brown for matching oxidised copper fittings. |
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| Dark Florentine Bronze |
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Copper electroplated and dipped,
dark brown for matching oxidised copper fittings. |
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| Copper |
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Electroplated, used as a base
for nickel or for improved conductivity. |
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| Tin |
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Electroplated, used to
facilitate soldering. |
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| Nickel |
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Electroplated over copper, hard
bright silver finish. Often used in electrical appliances
and areas of condensation - not sacrificial. |
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| Chrome |
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Electroplated over nickel, very
hard, bright, reflective finish; easy to clean or polish.
Used in heavy condensation areas - not sacrificial. |
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| With sacrificial protective
coatings, the thicker the deposit, the longer the protection;
however, there are practical and economic limitations to the
thickness applied. |
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| Zinc electroplating can
provide thicknesses from a negligible flash of colour, for
appearance, through normal commercial coatings of 3-5 microns
(µm), to specified heavy coatings up to 12 microns (0.0005 in).
Electroplating does not give an even cover; thicker
concentration of deposit occurs on corners, points, thread
crests and thinner concentrations on thread flanks and roots.
This may cause thread galling on coatings above 8 microns
average and adjustment by over - tapping of the nut may be
required. |
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| Hot dip galvanising will
allow much heavier coatings, the normal commercial coating is
approximately 50 microns (µm), which necessitates the
over-tapping of the mating thread and is the maximum practical
to avoid serious compromise of the fastener's strength. Unlike
electroplating, the concentration of deposits is in the thread
roots and internal corners. For this reason, thread diameters of
less than M10 are not normally galvanised unless a subsequent
light re-roll of the thread is performed. |
| Nuts supplied with galvanised bolts
will have over-tapped threads to allow for the galvanised
build-up on bolt threads and to reduce assembly galling. |
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| Mechanical coating will
result in a more even deposit and the point of over-tapping will
be raised above 15 µm. Comparable thicknesses can be achieved
but costs are generally much higher. |
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| High tensile or hardened fasteners
above PC 8.8 or SAE Grade 5 are susceptible to hydrogen
embrittlement in the cleaning and coating process, particularly
electroplating. They absorb hydrogen atoms which concentrate in
areas of stress, causing minute cracks which can suddenly and
violently fail in service. |
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| To avoid this potential, the
hydrogen atoms can be diffused by baking the product immediately
after plating, prior to chromating at a temperature of 190°C to
210°C for a period depending upon the grade and size of the
product. |
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| For this reason, it is most
unadvisable to plate PC 10 or SAE Grade 8 and higher products
after purchase, unless the Plater is also able to perform and
guarantee the de-embrittlement process. |
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Service life of coatings prior to
first signs of corrosion will vary considerably depending upon
thickness and environment.
Experience suggests the following: |
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| Stainless Steel is self protecting,
as shown below |
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| Apart from general corrosion
(rust) caused by exposure of uncoated materials there are
several other types of corrosion which effect ferrous and
non ferrous materials. |
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They include:
2. Pitting
3. Crevice Corrosion
4. Stress Corrosion Cracking
5. Galvanic Corrosion |
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| - Further information on types
2,3 and 4 may be available in future publications. Please
consult your local representative if required. |
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| - A selection chart to provide
guidance on limiting the effects of type 5 - is referred to
below |
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| Galvanic
Corrosion |
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In addition to corrosion being
caused by exposure of uncoated materials, it is also caused
or enhanced by the combination of dissimilar or incompatible
materials. |
The following chart gives guidelines
for the selection of materials or finishes based on this
galvanic action: |
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| Key |
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A |
The corrosion of the base metal is
not increased by the fastener. |
D |
The plating on the fastener is
rapidly consumed, leaving the bare fastener material. |
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B |
The corrosion of the base metal is
marginally increased by the fastener. |
E |
The corrosion of the fastener is
increased by the base metal. |
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C |
The corrosion of the base metal may
be markedly increased by the fastener. |
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NR Not recommended. |
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