Corrosion handbook

32 06/21
Zn-alloy coatings like ZM have significantly reduced corrosion rates in atmospheric
applications. For lifetime estimation of such coatings the DIN 55634 provides a
useful selection table for general guidance. It is based on the technically sound
assumption that ZM coatings have half the corrosion rate compared to pure Zn
coatings.
Timespan until start of red rust (per exposure class)*
Coating Thickness C2 C3 C4 C5-I C5-M
(μm) L M H L M H L M H L M H L M H
Zinc
Z140 10 x x
Z275 20 x x x
Z600 42 x x x x x x x x x x
ZM
ZM140 11 x x x x
ZM275 21 x x x x x x x
ZM310 24 x x x x x x x x
* L = 2–5 years M = 5–15 years H = ˃ 15 years
4.3 Assessment of corrosivity for stainless steel
products
Strictly speaking, the corrosivity categories (C-classes) according to the ISO 9223
standard are applicable only to zinc, carbon steel, aluminum and copper. The
different corrosion mechanism of stainless steel makes it necessary to work with
a system of classification different to that used for zinc. Stainless steel is generally
stable against humidity and water when no corrosive contaminants are present. The
breakdown of the passive layer by substances like chlorides, however, has to be
taken into account specifically and limits the use of certain grades of stainless steel.
In contrast to zinc, the influence of pure humidity without other contaminants
is negligible for the corrosion of stainless steel. For stainless steel it is more
important to consider the effect of chlorides and of rain or other moving water
in washing off the corrosion products and corrosive substances. In many
applications the effect of this washing makes the use of weaker grades of
stainless steel possible. For applications like roofs and the facades of buildings
this can be an option. However, in typical applications where fasteners and
installation products are used there can be areas sheltered from the rain.
International (Eurocode 3, EN 1993-1-4 draft version) and national standards and
guidelines (such as the German technical approval DIBt Z.30.3-6) usually work
with a special scoring system when evaluating the suitability of certain stainless
steel grades.
This system takes the risks presented by the main influencing factors into
account, from which a corrosion resistance factor (CRF) is calculated.
Each risk factor (chlorides, sulfur dioxide, washing effect) is linked to a certain
number of points.
The CRF depends on the severity of the environment and is calculated as follows:
CRF = F1 + F2 + F3, where
• F1 = Risk of exposure to chlorides from salt water or de-icing salts
(categories ranging from +1 to -15, mainly determined by the distance
from the coast or roads, where de-icing salts are used)
• F2 = Risk of exposure to sulfur dioxide
(categories ranging from 0 to -15, determined by the average sulfur dioxide
concentration)
• F3 = Cleaning regime or exposure to washing by rain
(categories ranging from +1 to -7)
Table 4: Expected service life of Zn and ZM coatings