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What is corrosion?
Steel is dissolved in salt and air containing water while oxygen is reduced and hydrogen is evolved, and forms iron oxide/hydroxide which precipitates as a voluminous corrosion. This is rust. Once the surface is oxidized, iron oxide layers are not protecting against further corrosion.

How to fight corrosion?
With cathodic corrosion protection of steel, this is the typical method to protect steel and other iron material.

Types of coating

The types of coating for the steel nuts and bolts are divided based on the process of application.


They are:


- Electrolytic coatings (galvanic)

- Chemical coatings

- Mechanical coatings

- Hot dip coatings


Electrolytic coatings (galvanic)

Such coatings are obtained by the immersion of the material in an electrolyte solution containing zinc salts. A flow of electrical current is created between the workpiece and the solution to deposit the metallic zinc on the surface of the piece itself.


Thickness of galvanic coatings:

The thickness of electroplated coatings (galvanic) chemical mechanics are of the order of 3-20 microns.
The finishes are commonly used are the following: white galvanization (5, 8, 12 micron), black galvanization, tropicalization, polishing and nickel plating.


Chemical coatings (s/c UNI 3740/6)

The chemical coatings used for the bolts are of two types:


- Phosphatic

- Metal based and organic binders based


Mechanical coatings (s/c UNI 3740/6)

These coatings, obtained by the mechanical action of a powder consisting of Zn, Sn, or Al or combinations thereof, are recommended for high strength bolts with a tensile strength Rm greater than or equal to 1040 N/mm2 since the procedure does not cause hydrogen embrittlement.


Hot dip coatings

Such coatings, obtained by immersion in a bath of molten zinc, are not recommended for the nuts and bolts of small size and/or with tolerances of a precise degree, they are instead recommended for the bolts used in environments with a high rate of corrosion.


Thickness of hot dip coatings:


The thicknesses of hot dip coatings are of the order of 70-100 micron and they should be done with the thread undersized, generally process again the thread of the nut after its coating.

zincatura-f1 tabella-tt-galvanici-ctt-galvanicic

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