Sendzimir galvanising, Electrogalvanizing, Electrogalvanized bichromate, Hot-dip galvanising, Satainless steels, A 304 Stainless stell, Tests
Corrosion
is an electro-chemical phenomenon that arises from contact between two different
metals, or a metal and the environment (air, water, gases) surrounding it.
All metals are liable to corrosion to some extent. The end result of the
corrosion process is the total destruction of the metal affected.
On view of the high costs stemming from corrosion, there are various industrial
coatings that can be applied on the surface in order to retard the appearance
of corrosion.
The duration of the protection achieved depends on the thickness of the
protective coating, expressed in microns (1 micron = 0.001 mm), the type
of coating used, and the aggressiveness of the environment.
|
Thickness
in microns
|
|
30
|
|
40
|
|
80
|
|
100
|
|
Estimated
service life in years of the protective coating in atmospheres...
|
|
Industrial*
|
Marine
|
Urban
|
Rural
|
|
-
|
4-12
|
5-10
|
13-28
|
|
-
|
6-18
|
7-15
|
20-40
|
|
7-14
|
11-33
|
13-25
|
-
|
|
9-18
|
14-40
|
15-35
|
-
|
* In the case of steel, the most highly developed industrial process is zinc coating.
All the steel products made by PEMSA are protected by one of the following coatings:
>>G.S.
Sendzimir galvanising UNE 36.130.EN.10142:1980/A1 (NOT TO BE CONFUSED WITH
HOT-DIP GALVANIZING):
>Used solely for protecting sheet metal of below 3 mm in thickness.
>Produced by dipping the rolls of steel in a zinc bath at 450 ºC.
>This technique is widely used in the manufacture of bar-cut parts and
stamped parts. The cuts arising in the machining are partially regenerated
by the galvanic pair formed by the steel and the zinc.
>The thickness of the protecting coating is from 8 to 15 microns.
>Recommended for indoor installations in a dry atmosphere with no aggressive
contaminants. back
>>E.Z. Electrogalvanizing (white colour)
UNE 112-036-ISO 2081:
>This involves protecting parts made of iron and steel electrolytically
using zinc..
>The thickness of the coating applied varies from 8 to 12 mm - see table
II. The Standard recommends applying a chrome-conversion layer (UNE 112-050).
>Recommended for indoor installations in a dry atmosphere with no aggressive
contaminants. back
>>Electrogalvanized bichromate
(yellow colour) UNE 112-050 ISO 4520:
>This involves the above process plus subsequent treatment with chromium
salts that improve the anti-corrosion performance - see table II.
>The thickness of the coat applied is 8-12 mm. .
>The process as used by PEMSA involves no CYANIDE.
>Recommended for use in dry, contaminant-free atmospheres.
>Better performance than G.S. and E.Z. back
>>G.C. Hot-Dip galvanising UNE-EN ISO
1461-99:
>Used for all kinds of products.
>It involves immersing the individual pieces to be protected into a bath
full of zinc at 450 ºC after the parts have been machined.
>The average thickness of the protective coating is 70 microns.
>Recommended for outdoor, marine, rural, industrial and aggressive indoor
installations. back
>>STAINLESS STEELS:
>These are carbon steels alloyed with chromium, nickel, magnesium and
molybdenum. The best-known ones are AISI 304 (18/8) and AISI 316.
>Contrary to widespread belief, these steels ALSO SUFFER CORROSION, though
their performance when properly handled is generally much better than any
type of protective coatingr.
>Their specific behaviour is the result of the chromium and nickel spontaneously
generating a film of chromium oxide in the presence of an oxidising environment,
this film then preventing subsequent oxidisation. The film is very delicate,
and can be broken by blows, machining, welding etc.
>Stainless steels are easily CONTAMINATED, thus losing their properties,
when they are machined with tools that use other types of steel. In such
cases, the protective film must be generated by a chemical-attack process.
>To solve this problem, after the manufacturing work (punching, welding),
PEMSA subjects all its REJIBAND INOX
rack and accessories to a special process for regenerating the protective
layer known as THERMICRON. back
>>A 304 STAINLESS STEEL AISI-304. Suitable
for:
>Fresh
water.
>Industrial atmosphere.
>Marine atmosphere.
>Normal chemical industry.
>FOOD INDUSTRY (except high temperatures).back
>>Laboratory tests carried out
in a salt-spray chamber in accordance with ASTM-B117 produced the following
results:
|
TABLE
II
|
Corrosion
resistance in hours
|
|
|
White
|
Red
|
|
| Electrolytic zinc coating, birchromate (yellow) |
198
|
428
|
| Electrolytic zinc coating (white) |
21
|
290
|
| Hot-dip galvanizing |
21*
|
1.250
|
| Stainless steel AISI 304 | ||
* The oxidation stopped when in contact with circulating air.
An approximate way of finding out how laboratory values translate into real-world values is: 1 laboratory hour = 1 week (In the case of an industrial marine atmosphere, divide the above results by 2). For guideline purposes, since each installation is different, the following protective coatings can be advised.
TABLE III: TABLE FOR CHOOSING SURFACE TREATMENTS IN VIEW OF THE ENVIRONMENT
|
TIPE
|
E.Z.
|
Z.B.
|
G.S.
|
G.C.
|
INOX
304
|
|
Indoor
|
B
|
B
|
B
|
-
|
-
|
|
Indoor,
near sea
|
M
|
P
|
M
|
B
|
B
|
|
Outdoor,
dry climate
|
P
|
B
|
P
|
B
|
B
|
|
Outdoor,
near sea
|
M
|
M
|
M
|
P
|
P
|
|
Coast
|
M
|
M
|
M
|
P
|
P
|
|
Sulphurous
|
M
|
M
|
M
|
P
|
P
|
|
Mineral
acids
|
M
|
M
|
M
|
P
|
P
|
|
Ammonium
|
M
|
M
|
M
|
P
|
P
|
|
Caustic
soda
|
M
|
M
|
M
|
P
|
P
|
|
Organic
acids
|
M
|
M
|
M
|
P
|
B
|
|
Hydrocarbons
|
M
|
M
|
M
|
P
|
B
|
|
Chlorine
|
M
|
M
|
M
|
M
|
P
|
|
Food
industry
|
P
|
P
|
P
|
P
|
B
|
|
Mechanical
erosion
|
M
|
M
|
M
|
B
|
-
|
*B: Good P: Possible M: Poor
