Corrosion and corrosion protection
© – A Concept for Progress and Quality 2
1 The material steel and its characteristics __________________________________________________ 3 1.1 Characteristics ________________________________________________________________ 3 1.2 Alloy elements_________________________________________________________________ 3 1.3 Surface ______________________________________________________________________ 4 1.4 Steel strip/slit strip ______________________________________________________________ 5 1.5 Hot rolling/cold rolling ___________________________________________________________ 5 1.6 Steel types and material frequently used at MÜPRO ___________________________________ 5
2 The fundamentals of corrosion____________________________________________________________ 6 2.1 Corrosion system ______________________________________________________________ 6 2.2 Fundamental processes of corrosion _______________________________________________ 6 2.3 Corrosion effects _______________________________________________________________ 8 2.4 Types of corrosion______________________________________________________________ 9
2.4.1 Uniform surface corrosion ___________________________________________________ 9 2.4.2 Pitting corrosion __________________________________________________________ 10 2.4.3 Shallow pit corrosion ______________________________________________________ 10 2.4.4 Crevice corrosion _________________________________________________________ 10 2.4.5 Contact corrosion _________________________________________________________ 11 2.4.6 Stress corrosion cracking___________________________________________________ 13 2.4.7 Corrosion fatigue cracking __________________________________________________ 13 2.4.8 Hydrogen induced internal cracking __________________________________________ 14
3 Corrosion protection systems____________________________________________________________ 15 3.1 Corrosion protection – general information __________________________________________ 15
3.1.1 Definition ________________________________________________________________ 15 3.1.2 Fog test DIN 50021 / ISO 9227 / ISO 7253______________________________________ 15 3.1.3 Duration of corrosion protection according to coating variant _____________________ 16
3.2 Corrosion classes _____________________________________________________________ 17 3.3 Types of corrosion protection ____________________________________________________ 17
3.3.1 Electrolytic galvanisation ___________________________________________________ 18 3.3.2 Sendzimir galvanisation (strip galvanisation) ___________________________________ 20 3.3.3 Hot-dipped galvanising (discontinuous galvanising) _____________________________ 21 3.3.4 Zink-nickel coating ________________________________________________________ 24 3.3.5 Special forms of the coating ________________________________________________ 25
4 Application table ________________________________________________________________________ 29
List of figures _______________________________________________________________________________ 31
Corrosion and corrosion protection
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1 The material steel and its characteristics
1.1 Characteristics
According to the classic definition, steel is an iron-carbon alloy with a mass proportion of iron that is
greater than that of any other element, and with a carbon content that generally is less than 2 % by
weight C. Today steels are considered to be iron-based alloys that can be plastically deformed.
Steels are the most widely used metallic materials. Through alloying with carbons and other elements in
combination with heat and thermal-mechanical treatment, the characteristics of steel can be adapted for
a broad spectrum of applications.
Worldwide steel production in millions of tons1:
Year Production
1998 777
2000 848
2008 1.330
1.2 Alloy elements
Steel materials are divided into groups according to alloy elements, the structural components and the
mechanical properties. The groups are differentiated according to alloy content as: Unalloyed, low-alloy,
and highly-alloy steels. The influences of the alloy components are described in the following table.
1 Compare Wikipedia, referenced on 18.01.2010
Corrosion and corrosion protection
© – A Concept for Progress and Quality 4
Fig. 1 Influence of the alloy elements (selection)2
1.3 Surface
Because steel has a high affinity for oxygen, oxidation easily occurs. Untreated steel quickly starts to
rust in conjunction with oxygen and moisture-enriched atmospheres . This is why an additional surface
treatment is strictly required. Every year decomposition of iron materials to rust due to air and water
causes billions of Euros of damage worldwide.
Fig. 2 Corrosion
2 Compare Europa Lehrmittel, Tabellenbuch Metall, 1997, S. 116
Characteristics that are influenced by alloy elements
Cr Ni Al W V Co Mo Si Mn S P
Tensile strength
Apparent limit of elasticity
Impact value
Wear resistance
Hot deformation
Cold deformation
Machineability
Heat-resistance
Corrosion resistance
Tempering temperature Hardenability, heat-treating quality ,
Nitriding quality
Weldability
Alloy elements
Increase Decrease Negligible influence
Corrosion and corrosion protection
5 © – A Concept for Progress and Quality
1.4 Steel strip/slit strip
Steel strip is the term for flat products that are produced in the widths suitable for galvanization or
processing.
Products made of galvanised steel strip are galvanized on the longitudinal edges.
Slit strip is the term that designates strips that are split by dividing flat products longitudinally to the
width that is suitable for galvanization or processing.
Products made of galvanised slit strip are galvanized on the longitudinal edges.
1.5 Hot rolling/cold rolling
Rolling is a deformation process that occurs after primary forming (continuous casting). The rolling stock
(slabs or billets) is reduced to a specified thickness in the roll gap through application of pressure. There
are changes in length and width due to the law of volume constancy.
Cold rolling is executed after hot rolling.
The delimitation between hot rolling and cold rolling is determined by the temperature. For hot rolling the
rolling temperature is always above the recrystallisation temperature, for cold rolling it is always below
the recrystallisation temperature.
Recrystallisation temperature is the temperature at which a material completely recrystalises, i.e.
hardening and tensions are dissipated. Frequently the rule of thumb estimate is 40 or 50% of the
absolute melting temperature.
1.6 Steel types and material frequently used at MÜPRO
S 235 JR (formerly St 37) General structural steel, e.g. Mounting Angle, Cantilever bracket
S 355 J2 (formerly St 52) General structural steel with increased strength, e.g. pipe supports for
MÜPRO Maritime
DD 11 (formerly StW 22) Bare slit strip, e.g. Optimal Junior, Optimal, Single Bossed Clamp
DX 51 D Hot-dip galvanised slit strip, e.g. MPC, MPR Support Channels
S 185 (formerly St 33) Hot-dipped galvanised steel strip, e.g. industrial pipe clamps
Corrosion and corrosion protection
© – A Concept for Progress and Quality 6
2 The fundamentals of corrosion
The term corrosion3 designates a process that occurs between a material and its environment. It is an
interaction that can result in a change of characteristics of the material, and thus significant impairment
of its function, the environment or of the technical system of which the material is only one part. To the
observer this process can be perceived through a corrosion effect that is a visible partial result of the
course of the reaction.
2.1 Corrosion system
A corrosion system consists of the material, the corrosion medium, and all associated phases, with
chemical and physical variables that influence the corrosion.
Coatings, surface layers, or additional electrodes can also be part of the environment.
Fig. 3 Corrosion system
2.2 Fundamental processes of corrosion
In the schematic diagram a drop of water surrounded by air is on an iron surface. According to the
standard electrode potential of the elements, the positively charged iron ions diffuse in the aqueous
environment, the electrons remain in the metal and charge it negatively (1).
However in general the negative charging of the metal, and the boundary layer of positively-charged
iron ions above the iron surface prevent a fast transformation with protons: Pure water does not corrode
the ferrous metal.
3 Definition of terms in accordance with DIN EN ISO 8044
Material Medium (environment)
Phase limit
Corrosion and corrosion protection
7 © – A Concept for Progress and Quality
However if oxygen is present, it can take over transport of the electrons. It diffuses into the water
droplets from the outside. The concentration differential in the water droplets now generates a potential
difference between (2) and (3). Thus the anodic area (2) and the cathodic area (3) form a galvanic cell
with the water as electrolytes; a redox reaction occurs. The electrons react with water and oxygen to
form hydroxide ions (3).
The hydroxide ions form iron(II)-hydroxide (4) with the iron ions (4). However this is transformed in the
presence of water and air so that iron III ions occur. Together with the hydroxide ions, with this second
redox reaction, rust-brown iron(III)-hydroxide is formed from the low solubility iron(III)-oxide-hydroxide
through water delivery, that is deposited on the iron surface at (5).
The initially formed mixture of iron(II)-hydroxide and iron(III)-hydroxide, thus is through partial water
delivery ultimately becomes a resistant mixture of iron(ll)-oxide, iron(III)-oxide, which is referred to in the
vernacular as rust.
Fig. 4 Schematic diagram of the corrosion process4
4 Illustration according to Rio GmbH (www.rio.de)
-
- -
-
-
- - - --
-
++
+ +
-
3
2
31
5 5
44
--
-- --
--
--
-- -- -- ----
--
++++
++ ++
--
33
22
3311
55 55
4444
Corrosion and corrosion protection
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2.3 Corrosion effects
Fig. 5 Changes caused by corrosion in any part of the corrosion system
Fig. 6 Corrosion effect5
Fig.7 Corrosion effect with impairment of the function of the material6
5 Illustration according to Rio GmbH (www.rio.de) 6 Illustration according to Rio GmbH (www.rio.de)
Cracks Pitting Shallow pitting
Surface corrosion
Inner corrosion
Cross section
Top view
Corrosion and corrosion protection
9 © – A Concept for Progress and Quality
2.4 Types of corrosion
Fig 8 Types of corrosion (excerpt)
2.4.1 Uniform surface corrosion
In the case of uniform surface corrosion, the material is removed virtually uniformly from the surface.
Fig. 9 Surface corrosion7
This type of corrosion can be effectively handled with a surface coating adapted to the conditions.
7 Steel sculpture in the park, "Am Warmen Damm", in Wiesbaden
Surface corrosion
Pitting corrosion
Shallow pit corrosion
Crevice corrosion
Contact corrosion
Hydrogen-induced inner cracking
Stress-cracking corrosion
Corrosion fatigue cracking
Local corrosion on the phase limit with
mechanical stress
Local corrosion in the inner material
Local corrosion on the phase limit without mechanical stress
Corrosion and corrosion protection
© – A Concept for Progress and Quality 10
2.4.2 Pitting corrosion
Pitting corrosion is a locally limited corrosion and has different manifestations. The typical situation is
that the depth of the pitting is usually greater than its diameter.
Fig. 10 Pitting corrosion8
2.4.3 Shallow pit corrosion
Shallow pit corrosion is also a form of local corrosion. In the case of non-uniform surface removal,
shallow pits are formed, which opposite to pitting corrosion, have a diameter that is greater than the
depth.
2.4.4 Crevice corrosion
Crevice corrosion is local corrosion in conjunction with gaps that run in or directly adjacent to a crevice
area that has formed between the metal surface and a different surface.
In the crevice the fluid exchange with the environment is restricted. These types of gaps are design-
occasioned or operation-occasioned. The corrosion mechanism essentially corresponds to that of pitting
corrosion. Crevice geometry and the types of crevice-forming materials are additional influence factors.
Since crevice corrosion occurs at significantly lower (weaker) levers of corrosion stress than does pitting
corrosion the occurrence of crevices should be avoided through design measures to the extent possible.
8 Illustration according to Rio GmbH (www.rio.de)
Corrosion and corrosion protection
11 © – A Concept for Progress and Quality
2.4.5 Contact corrosion
Contact corrosion, or bi-metal corrosion, is galvanic corrosion in which the electrodes are formed by
different metals.
Fig. 11 Contact corrosion
The metals are directly or conductively connected.
Both metals have a connection to the same electrolytes.
The area ratio of anodic to cathodic surface.
Area rule: A anode / A cathode >> 1 (favourable)
Corrosion and corrosion protection
© – A Concept for Progress and Quality 12
Example:
Small hot-dipped galvanised area with large stainless steel area:
S - strong corrosion on the hot-dipped galvanized components.
Material considered relative to contact corrosion
Are
a ra
tio
Mag
nesi
um a
lloy
Zin
c
Hot
-dip
ped
galv
anis
ed s
teel
Alu
min
ium
allo
y
Cad
miu
m
coat
ing
Str
uctu
ral s
teel
Low
-allo
y st
eel
Cas
t st
eel
Chr
ome
stee
l
Lead
Tin
Cop
per
Sta
inle
ss s
teel
Magnesium alloysmalllarge
SM
SM
SM
SM
SS
SS
SS
SS
SS
SS
SS
SS
Zincsmalllarge
ML
LL
ML
ML
SL
SL
SL
SL
SL
SL
SL
SL
Hot-dipped galvanised steel
smalllarge
ML
LL
ML
ML
SL
SL
SL
SL
SL
SL
SL
SL
Aluminium alloysmalllarge
ML
LM
LM
LL
ML L
SM M
SS S
SS
SM
Cadmium coatingsmalllarge
LM
LL
LM
LL
SL
SL
SL
SL
SL
SL
SL
SL
Structural steelsmalllarge
LL
LL
LL
LL
LL
ML
SL
SL
SL
SL
SL
SL
Low-alloy steelsmalllarge
LL
LL
LL
LL
LL
LL
LL
SL
SL
SL
SL
SL
Cast steelsmalllarge
LL
LL
LL
LL
LL
LL
ML
SL
SL
SL
S S
Chrome steelsmalllarge
LL
LL
LL
LL
LL
LL
LL
ML
ML
S SL
Leadsmalllarge
LL
LL
LL
LL
LL
LL
LL
LM
LL
LL
LL
Tinsmalllarge
LL
LL
LL
LL
LL
LL
LL L
LM
LL
Coppersmalllarge
LL
LL
LL
LL
LL
LL
LL L
M ML
SM L
Stainless steelsmalllarge
LL
LL
LM
LL
LL L
LL
LL M
LM
LM L
S = Strong corrosion of the material considered
M = Moderate corrosion of the material considered (in an extremely moist atmosphere)
L = little or no corrosion of the considered material
Material considered relative to contact corrosion
Are
a ra
tio
Mag
nesi
um a
lloy
Zin
c
Hot
-dip
ped
galv
anis
ed s
teel
Alu
min
ium
allo
y
Cad
miu
m
coat
ing
Str
uctu
ral s
teel
Low
-allo
y st
eel
Cas
t st
eel
Chr
ome
stee
l
Lead
Tin
Cop
per
Sta
inle
ss s
teel
Magnesium alloysmalllarge
SM
SM
SM
SM
SS
SS
SS
SS
SS
SS
SS
SS
Zincsmalllarge
ML
LL
ML
ML
SL
SL
SL
SL
SL
SL
SL
SL
Hot-dipped galvanised steel
smalllarge
ML
LL
ML
ML
SL
SL
SL
SL
SL
SL
SL
SL
Aluminium alloysmalllarge
ML
LM
LM
LL
ML L
SM M
SS S
SS
SM
Cadmium coatingsmalllarge
LM
LL
LM
LL
SL
SL
SL
SL
SL
SL
SL
SL
Structural steelsmalllarge
LL
LL
LL
LL
LL
ML
SL
SL
SL
SL
SL
SL
Low-alloy steelsmalllarge
LL
LL
LL
LL
LL
LL
LL
SL
SL
SL
SL
SL
Cast steelsmalllarge
LL
LL
LL
LL
LL
LL
ML
SL
SL
SL
S S
Chrome steelsmalllarge
LL
LL
LL
LL
LL
LL
LL
ML
ML
S SL
Leadsmalllarge
LL
LL
LL
LL
LL
LL
LL
LM
LL
LL
LL
Tinsmalllarge
LL
LL
LL
LL
LL
LL
LL L
LM
LL
Coppersmalllarge
LL
LL
LL
LL
LL
LL
LL L
M ML
SM L
Stainless steelsmalllarge
LL
LL
LM
LL
LL L
LL
LL M
LM
LM L
S = Strong corrosion of the material considered
M = Moderate corrosion of the material considered (in an extremely moist atmosphere)
L = little or no corrosion of the considered material
Fig 12 Area ratio9
9 Illustration according to Institut Feuerverzinken - www.feuerverzinken.de
Corrosion and corrosion protection
13 © – A Concept for Progress and Quality
2.4.6 Stress corrosion cracking
Stress corrosion cracking is chemical and / or electro-chemical corrosion of a metal under the
concurrent effect of a corroding agent and static tensile stress. Stress corrosion cracking is particularly
dreaded in the construction industry because it can only be detected with difficulty and can result in an
abrupt failure.
Fig. 13 Stress corrosion
2.4.7 Corrosion fatigue cracking
Corrosion fatigue cracking is chemical and / or electro-chemical corrosion of a metal under the
concurrent effect of a corroding agent and alternating mechanical stress.
Fig. 14 Corrosion fatigue
Corrosion and corrosion protection
© – A Concept for Progress and Quality 14
Fig. 15 Corrosion fatigue (practice)
2.4.8 Hydrogen induced internal cracking
Hydrogen induced internal cracking, also referred to as hydrogen embrittlement occurs through a
change in the grain boundaries of the steel.
Steel is often affected by embrittlement if it has been in contact with hydrogen for a longer period of
time. Among steels however the austenitic steels (e.g. CrNi steels) are an exception. For the most part
these steels are not sensitive to hydrogen embrittlement and are among the standard materials for
hydrogen technology.
Hydrogen embrittlement particularly occurs when welding and with electrolytic galvanising of steels with
high tensile strength (e.g. screws of strength class 10.9 and higher). Hydrogen is formed on the
cathodically switched steel and diffuses into the steel.
So that the screw can release the hydrogen again it must be immediately subjected to thermal treatment
for several hours at approx. 200 - 300 C (low-hydrogen annealing).
Corrosion and corrosion protection
15 © – A Concept for Progress and Quality
3 Corrosion protection systems
Corrosion protection systems promote the safety and extend the service life of metallic components and
workpieces in the context of the environmental influences.
3.1 Corrosion protection – general information
Corrosion protection is a change in a corrosion system that reduces corrosion damage.
Fig. 16 Changes in the corrosion system to prevent corrosion damage
3.1.1 Definition
The term corrosion protection designates all active or passive measures for protecting materials or
components from damage. Passive corrosion protection is the application of organic or inorganic
protective coatings. Active corrosion protection is the designation for all measures that exert a direct
influence on the corrosive medium (e.g. inhibitors in drinking water systems). Selection of the wrong
stainless steel quality can result in corrosion damage in specific cases. Even high-quality stainless
steels are not suitable for every application area! The effectiveness of a corrosion-protection process
can be determined comparatively through salt spray testing.
3.1.2 Fog test DIN 50021 / ISO 9227 / ISO 7253
In the chamber the sprayed salt solution produces a corroding atmosphere that generates a corrosive
action on the exposed parts.
Under these conditions the corrosion process accelerates and the coatings lose their corrosion
protection during the test. The parts become corroded more quickly than they would otherwise under
normal conditions in the application.
Materialselection
Material Medium (environment)
Phase limit
Elimination of corrosion-promoting
agents
Inhibitors
coatingsElectro-chemical
protection
Design measures
Corrosion and corrosion protection
© – A Concept for Progress and Quality 16
The duration of the test depends on the requirements of the coating. Because the concentration of the
aqueous salt solution, temperature, pressure, pH value must be maintained constant, the results can be
reproduced.
Fig. 17 Chamber for salt spray testing10
3.1.3 Duration of corrosion protection according to coating variant
Electrolyticgalvanisation
Hot-dipgalvanised
Zinc-nickel MCPS coating
Ho
urs
un
til
the
firs
t o
ccu
rren
ce o
f re
d r
ust
(s
alt
sp
ray
tes
t)
Powder coating Duplexpowder coating
250
500
750
1.000
1.500
1.250
Del
ta-T
on
eD
elta
-S
eal
Electrolyticgalvanisation
Hot-dipgalvanised
Zinc-nickel MCPS coating
Ho
urs
un
til
the
firs
t o
ccu
rren
ce o
f re
d r
ust
(s
alt
sp
ray
tes
t)
Powder coating Duplexpowder coating
250
500
750
1.000
1.500
1.250
Electrolyticgalvanisation
Hot-dipgalvanised
Zinc-nickel MCPS coating
Ho
urs
un
til
the
firs
t o
ccu
rren
ce o
f re
d r
ust
(s
alt
sp
ray
tes
t)
Powder coating Duplexpowder coating
250
500
750
1.000
1.500
1.250
250
500
750
1.000
1.500
1.250
Del
ta-T
on
eD
elta
-S
eal
Fig. 18 Duration of corrosion protection according to coating variant
10 Illustration from ERICHSEN GmbH & Co. KG - www.erichsen.de
Corrosion and corrosion protection
17 © – A Concept for Progress and Quality
3.2 Corrosion classes
The EN ISO 12944 describes the different protection measures offered by coating systems. Ambient
conditions, design rules and coating systems are described and classified is this standard. Ambient
conditions can be divided into 6 categories based on this benchmarking data. This division helps in the
selection of the suitable corrosion protection system. The first step is differentiated as:
C1 - insignificant
C2 - low
C3 - moderate
C4 - high
C5-I - very high - industry
C5-M - very high - marine
In addition in the standard the duration of protection is grouped in 3 timeframes:
S - Short - 2 to 5 years
M - Medium - 5 to 15 years
L - Long - more than15 years
If the requirements imposed on the corrosion protection have been defined in accordance with the
standard, you can select the suitable protective process based on the classification.
3.3 Types of corrosion protection
MÜPRO offers corrosion-protection processes that are suitable for different components:
Electrolytic galvanizing Application of a homogeneous zinc coating through electrochemical processes
Continuous strip galvanising / Sendzimir galvanising Pre-galvanised strip material
Hot-dip galvanising Application of a zinc coating in a zinc bath (temperature ~450°C)
Electrolytic surface coating on a Zinc-nickel basis
Micro-layer Corrosion Protection This coating consists of a base coat and a top coat
Powder coating - polyester powdered based coating in a single-layer process
Duplex powder coating applied in a two-coat process on the basis of epoxy-resin polyester powder
Corrosion and corrosion protection
© – A Concept for Progress and Quality 18
3.3.1 Electrolytic galvanisation
Electrolytic galvanising - overview
Characteristics Application of zinc coat via electrolytic processes Coating thicknesses can be between 3 - 30 µm Large or bulky parts can only be electrolytically galvanized under certain conditions due to their geometry Surface is homogeneous and glossy in fresh status
Procedure Metallically clean steel parts (is achieved by pickling in hydrochloric acid) are suspended as cathode (-pole) in a zinc bath (+pole) When the power is supplied zinc precipitates on the steel and forms a protective layer The layer thickness is regulated by time period and current strength
Advantages For scratches/cut edges, a protection in certain dimensions is still provided (cathodic protective effect: Scratch end points make zinc ions available for protection); distances of more than 2-3 mm should not be bridged. Precisely adjustable layer thicknesses, i.e. metal is saved. Rework of thread, fit points, etc is not required. Heat has no effect on the base material, i.e. no deformation, formation of brittle diffusion layers (hard zinc), etc. Additional chromating is possible, increases the corrosion protection and changes the surface colour - Blue chromated - MÜPRO standard - Yellow chromated - is no longer offered, contains chrome VI - Black chromated - maximum corrosion protection for electrolytically galvanised products
Area of application Particularly suitable for indoor areas without corrosive atmosphere Not suitable for outdoor areas
MÜPRO products Characteristics Example of designation
MÜPRO products are usually electrolytically galvanised Layer thicknesses between 8-12 µm "Electro. Zn 8' means electrolytically galvanised, layer thickness 8 µm
Fig. 19 Electrolytically galvanised products from MÜPRO
Corrosion and corrosion protection
19 © – A Concept for Progress and Quality
MPC Wall Hanger Brackets before galvanising
Fig. 20 MPC Wall Hanger Brackets before galvanising
MPC Wall Hanger Brackets after galvanising
Fig. 21 MPC Wall Hanger Brackens after galvanising
Corrosion and corrosion protection
© – A Concept for Progress and Quality 20
3.3.2 Sendzimir galvanisation (strip galvanisation)
Electrolytic galvanising - overview
Characteristics Layer thicknesses between 10 and 40 µm Cut edges that are generated after galvanising, and initially zinc-free Cathodic protection effect also deposits zinc on the edges over time Cut edges can become slightly rusty over time, however the service life and serviceability are not influenced!
Procedure In the galvaniser's shop the sheet metal is first heated to approx. 800°C under inert gas in a continuous process (named after Tadeusz Sendzimir). Annealing of the steel is primarily used for adjusting the desired mechanical characteristics of the steel through recrystallization Subsequently the steel strip is cooled to temperatures of approx. 460°C, run - still under inert gas - diagonally into the zinc bath and then it routed upwards in the zinc bath through a deflection roller. When steel strip emerges, excess zinc is stripped off of the sheet metal by wide air nozzles so that a defined thickness of zinc remains on the sheet metal
Advantages Sendzimir galvanised input stock offers good corrosion protection without further treatment after the manufacturing process At material thicknesses up to max. 3 mm thicknesses the cut edges are protected from strong corrosion through the cathodic protective effect of the zinc
Area of application Interiors without a particularly corrosive atmosphere Not suitable for filigreed products or thread
MÜPRO products Characteristics Example of designation
Zinc layer thickness for MÜPRO products: 10-20 µm
Fig. 22 Sendzimir galvanised products from MÜPRO
Corrosion and corrosion protection
21 © – A Concept for Progress and Quality
3.3.3 Hot-dipped galvanising (discontinuous galvanising)
Hot-dip galvanising (discontinuous galvanising) - overview
Characteristics Zinc layer thicknesses are between 30 and 100µm depending on material thickness, immersion time, and reaction responsiveness of the steel
Procedure Immersion of the parts in a zinc bath (temperature 440-460°C, for high-temperature galvanizing to approx. 520°C) During the galvanisation process due to a reciprocal diffusion of the fluid zinc with the steel surface, a coating of differently composed iron-zinc alloys is formed on the steel part After removal from the zinc bath a so-called pure zinc coat forms on the surface Components are hung up or centrifuged to remove the " excess" zinc
Advantages With hot-dipped galvanising a pure zinc coat occurs that offers the longest corrosion protection The zinc coat is applied in a high-temperature zinc bath (~450°C) DIN EN IS0 1461 delineates the minimum layer thicknesses Due to the manufacturing procedure, smaller bores, thread, etc can be rendered unusable In this case as well, for small scratches/cut edges a cathodic protective effect occurs in certain dimensions
Area of application Suitable for outdoor implementation
MÜPRO products Characteristics Example of designation
Zinc layer thickness for MÜPRO products: 10 - 85µm Hot-dipped galvanised DIN EN IS0 1461
Fig. 23 Hot-dipped galvanised (discontinuously galvanised) products from MÜPRO
Corrosion and corrosion protection
© – A Concept for Progress and Quality 22
MPC Channels before hot-dip galvanising
Fig. 24 MPC Support Channels before galvanising11
MPC Channels after hot-dip galvanising
Fig. 25 MPC Support Channels after galvanising
11 Illustration Wiegel Feuerverzinken - www.wiegel.de
Corrosion and corrosion protection
23 © – A Concept for Progress and Quality
Th
ickn
ess
of
the
zin
c co
atin
g i
n µ
m
Duration of protection in years (average values)
200
150
100
50
0 10 20 30 40 50 60 70 80
Industrial a
ir Ocean air
City air
Country air
Interiors
Th
ickn
ess
of
the
zin
c co
atin
g i
n µ
m
Duration of protection in years (average values)
200
150
100
50
0 10 20 30 40 50 60 70 80
Industrial a
ir Ocean air
City air
Country air
Interiors
Fig. 26 Duration of protection of hot-dip galvanising12
The duration of protection of hot-dip galvanising is highly dependent on the ambient conditions. A basic
overview is provided in the graphic. It shows the duration of protection based on the layer thickness of
the zinc coating and the environment.
12 Illustration according to Institut Feuerverzinken - www.feuerverzinken.de
Corrosion and corrosion protection
© – A Concept for Progress and Quality 24
3.3.4 Zink-nickel coating
Zinc-nickel coating - overview
Characteristics Application of an alloy coating consisting of 85-90 % zinc and 10-15% nickel through electrolytic processes Layer thicknesses between 5 and 15 µm Highest level of corrosion protection even at layer thicknesses from 5 µm Ideal for small parts and all types of thread No contact corrosion when installed with aluminium Can be subjected to high levels of thermal stress to 180°C
Procedure Metallically clean steel parts are suspended as cathode (-pole) in an alkaline zinc-nickel bath (+pole) Through the supply of power the zinc-nickel alloy precipitates on the steel and forms a protective layer
Advantages Finishing of components to meet rigorous corrosion protection requirements Particularly well-suited for finishing small parts and threaded parts If there are scratches/cut edges, protection in certain dimensions is still provided (cathodic protective effect - scratch end points make zinc ions available for protection), distances of more than 2-3 mm should not be bridged Resistance in accordance with salt spray test as specified in DIN 50021 up to 1,000 hours
Area of application Implementation in outdoor areas
Fig. 27 Zinc-nickel coated products from MÜPRO
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3.3.5 Special forms of the coating
3.3.5.1 Powder coating (polyester powder basis)
Powder coating (polyester powder basis) - overview
Characteristics Application of a polyester-based top coat (single-coat process) Coating thicknesses can be between 60 - 80 µm Excellent spreading with excellent edge coverage Resistance in accordance with salt spray test as specified in DIN 50021 up to 500 hours
Procedure Electrostatically charged polyester powder is applied to the workpiece via a spray process An electrical field is generated between the earthed, conductive workpiece and the spray device The powder particles follow this field and remain bonded to the workpiece surface Subsequently the workpieces are heated in the oven at approx. 90°C; a durable surface is produced Threaded parts cannot be coated with an appropriate layer thickness
Advantages Application of an abrasion-resistant and chemical-resistant surface Highly resistant to weathering, e.g. UV-resistant Custom, extremely varied colour design of the surfaces based on the RAL colour chart Due to high layer thicknesses, not suitable for finishing small parts and threaded parts
Area of application Particularly well suited for visible areas or outdoor areas
Fig. 28 Powder-coated products from MÜPRO
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3.3.5.2 Micro-layer corrosion protection system (MCPS)
Microlayer corrosion protection system (MCPS) - overview
Characteristics High-level corrosion protection up to 1,000 hours in the salt spray test in accordance with DIN 50021 Improved weather-resistance, such as UV resistance Can be thermally stressed to 180°C, can be mechanically stressed High-level chemical resistance, high abrasive strength Extensive selection of colour schemes
Procedure Duplex process: Two coats are combined in the "division of labour principle" Base coat, e.g. "Delta protect KL100" or "Delta Tone 9000" Base coat, e.g. "Delta protect VH 300" or "Delta seal" Depending on the requirement, only a single-coat application can also be applied
Advantages For the most part the coatings are resistant to chemical corrosion Requirements imposed on the decorative and identifying design can be satisfied based on a variety of coating system colours Through a layer thickness that is adapted to the requirements in the range of 5 to 15 um, outstanding corrosion protection is ensured even for function parts with low tolerances, such as bolt thread Resistance in the salt spray test up to 1,000 hours Through a different combination of the individual coats a variety of corrosion protection requirements can be satisfied The configuration can only be executed after clarifying the customer's corrosion-protection requirements based on the project parameters
Area of application For products with a total length up to 3 m Massed-produced items - bolts, nuts, etc (pay attention to the thread size)
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Fig. 29 Micro-layer corrosion protection system - possible colours of the MCPS coating
Fig. 30 MCPS coated products from MÜPRO
DeltaTone 9.000 Silver
DeltaSeal Red
DeltaSeal Light blue
DeltaSeal Dark blue
DeltaSeal Green
DeltaSeal GZ Black
DeltaSeal Black
DeltaSeal Light grey
DeltaSeal Brown
DeltaSeal Silver
DeltaSeal GZ Silver
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3.3.5.3 Duplex powder coating
Duplex powder coating - overview
Characteristics Application of an epoxy base coat and a polyester powder coating as top coat for colour design (two-coat process) on a previously galvanised base material Coating thicknesses can be between 120 - 160 µm depending on the component geometry
Procedure Coating process corresponds to the powder coating process Work pieces with at least a 20 µm zinc layer thickness are coated with a closed pore epoxy-resin base coat Electrostatically-charged polyester powder is applied to the primed workpiece via a spray process
Advantages A surface is produced that is highly abrasion-resistant and can be subjected to high levels of chemical stress Resistance in accordance with salt spray test as specified in DIN 7253 up to 1,440 hours Closed-pore, surface that can be subjected to extreme chemical stress Custom, extremely varied colour design of the surfaces based on the RAL colour chart The combination of galvanisation and an additional powder coating enables a high level of corrosion protection that is configured for the requirements
Area of application Particularly well suited for visible areas or outdoor areas Implementation in swimming pools and in chloride-containing environments
Fig. 31 Duplex powder-coated products from MÜPRO
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4 Application table
The listing below shows the corrosion-protection systems that can be used in the application areas. The
list is provided as an orientation aid only. Selection of the suitable or necessary material/corrosion
protection relative to service life and safety, however depends on the ambient conditions and special
requirements and guidelines. Consequently in the chemical industry, for example, electrolytically
galvanised parts suffice in certain areas, while in other areas highly corrosion resistant materials are
required. Always check which material offers the greatest corrosion protection for the geographic
environment, on a case-by-case basis.
Fig 32 Corrosion protection systems according to area of application13
13 MÜPRO presentation
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Fig 33 Corrosivity categories14
14 Excerpt from EN ISO 12944
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List of figures
Fig. 1 Influence of the alloy elements (selection) ............................................................................................4
Fig. 2 Corrosion ...............................................................................................................................................4
Fig. 3 Corrosion system...................................................................................................................................6
Fig. 4 Schematic diagram of the corrosion process ........................................................................................7
Fig. 5 Changes caused by corrosion in any part of the corrosion system .......................................................8
Fig. 6 Corrosion effect .....................................................................................................................................8
Fig.7 Corrosion effect with impairment of the function of the material.............................................................8
Fig 8 Types of corrosion (excerpt) ...................................................................................................................9
Fig. 9 Surface corrosion ..................................................................................................................................9
Fig. 10 Pitting corrosion.................................................................................................................................10
Fig. 11 Contact corrosion ..............................................................................................................................11
Fig 12 Area ratio ............................................................................................................................................12
Fig. 13 Stress corrosion.................................................................................................................................13
Fig. 14 Corrosion fatigue ...............................................................................................................................13
Fig. 15 Corrosion fatigue (practice) ...............................................................................................................14
Fig. 16 Changes in the corrosion system to prevent corrosion damage........................................................15
Fig. 18 Duration of corrosion protection according to coating variant............................................................16
Fig. 19 Electrolytically galvanised products from MÜPRO ............................................................................18
Fig. 20 MPC Wall Hanger Brackets before galvanising.................................................................................19
Fig. 21 MPC Wall Hanger Brackens after galvanising...................................................................................19
Fig. 22 Sendzimir galvanised products from MÜPRO ...................................................................................20
Fig. 23 Hot-dipped galvanised (discontinuously galvanised) products from MÜPRO ...................................21
Fig. 24 MPC Support Channels before galvanising.......................................................................................22
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Fig. 25 MPC Support Channels after galvanising..........................................................................................22
Fig. 26 Duration of protection of hot-dip galvanising .....................................................................................23
Fig. 27 Zinc-nickel coated products from MÜPRO ........................................................................................24
Fig. 28 Powder-coated products from MÜPRO .............................................................................................25
Fig. 29 Micro-layer corrosion protection system - possible colours of the MCPS coating .............................27
Fig. 30 MCPS coated products from MÜPRO ...............................................................................................27
Fig. 31 Duplex powder-coated products from MÜPRO .................................................................................28
Fig 32 Corrosion protection systems according to area of application ..........................................................29
Fig 33 Corrosivity categories .........................................................................................................................30
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