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Corrosion and corrosion protection - Nordic groups are differentiated according to alloy content as: Unalloyed, low-alloy, and highly-alloy steels. The influences of the alloy components

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  • 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 MPRO ___________________________________ 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

    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

  • 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 MPRO

    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

    MPRO 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 w

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