Pitting corrosion - Corrosion protection
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The destruction can run purely chemically, in numerous cases also electro-chemically.
According to estimations of the Institute of Technology, Massachusetts there are annually billions of losses from corrosion to the national economy .
Everywhere you find metals, there is a problem of corrosion.
A galvanic element consists of three substantial parts:
- non precious metal (anode) delivers electrons, becomes the ion (negative potential)
- electrolyte contains ions in aqueous solution, makes their migration possible
- noble metal (cathode) remains unchanged, hydrogen becomes free (positive potential)
The non precious metal (sacrificial anode) corrodes, while the protected metal (e.g. zinc anodes with the outboard motor) remains intact: active corrosion protection.
The corrosion happens to the position of the metals in the galvanic electromotive series, the further it precedes in this row, the faster and more aggressively the process. For the extent of the corrosion is also the area proportion of the metals of substantial influence.
A large precious metal surface brought together with a small non precious, the corrosion will be serious, turned around the corrosion will be shown less clearly.
(in volts, related to a defined solution.
Alloys and other measuring solutions will change the values):
Aluminium - 1,66
Zink - 0,76
Chrom - 0,74
Eisen - 0,41
Nickel - 0,23
Zinn - 0,14
Blei - 0,13
Kupfer + 0,34
Silber + 0,80
Gold + 1,50
The
atmosphere is the most common corrosive means of attack for building and
materials. The rust is an unstable connection between oxygen, water and the metal,
salts accelerates the procedure.
Particularly with iron and steel, either surface demolition shows up or
in as pitting and pitting corrosion.
The atmospheric corrosion is dependent of the relative humidity and the
salt-content with sea-air:
In the USA e.g. retired airplanes are stored in an area with practically
no humidity (no electrolyte) .Even after 10 and more years there is no
rust at the metals.
Metal: Certain alloying elements work rust-restraining by a passivation (Developing
a close oxide film).
Despite of partly non precious potential certain metalls form under certain
conditions a protecting surface layer , which reduces the rate of corrosion
(copper, tin, chrome, nickel, zinc, aluminum).
Surface treatment:
The pretreatment before painting is just as important as the painting system
itself.
Pitting
corrosion is a manifestation of the corrosion, caused through
1. irregularities in the oxide skin:
The passivation will be destroyed after damages by scratches and impact
effects (often observed within the lower range of the mast)
or destruction of the oxide film by chlorine ions specialy in sea waters.
2. metal particles at the surface with different potential opposite the base
material
3. uneven ventilation or a different oxygen distribution at the surface:
e.g. water line corrosion due to more different
CO2 - concentration in the electrolyte sea water.
Oxygen-rich parts over water - cathodic reaction, oxygen-poor parts under
water - anodic reaction.
In
order to prevent or at least decrease an attack on metals and its alloys,
different measures are possible to their protection
passive: Surface treatment of the metal.
Chemical oxidation, anodic oxidation, metallic coats, color and lac painting,
email coats.
active:
Cathodic protection procedure.
The cathodic protection procedure uses - if an electrical auxiliary stream
is not put on - the building of a galvanic element by short circuit with
another non precious metal. The material which has to be protected forms
the cathode of the element.