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Page 1: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,
Page 2: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 3: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

Corrosion and corrosion protection

3 © – A Concept for Progress and Quality

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

Page 4: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 5: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 6: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 7: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 8: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

Corrosion and corrosion protection

© – A Concept for Progress and Quality 8

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

Page 9: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 10: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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)

Page 11: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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)

Page 12: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 13: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 14: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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).

Page 15: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

Page 16: Corrosion and corrosion protection - Nordic Pipenordicpipe.se/sites/nordicpipe.se/files/mupro_corrosion...The groups are differentiated according to alloy content as: Unalloyed, low-alloy,

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

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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

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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

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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

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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

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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

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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

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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

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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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