01 Corrosion Grade1

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Corrosion


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Definition of Corrosion

Corrosion is a reaction between

Material

and

Surrounding environment

under formation of corrosion products


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Production and

degradation of steel

Plates, pipes,

profiles, etc.

Raw materialIron ore Rust

Reaction between the material and

the surrounding environment takes place

The presence of water / humidity and Oxygen

is a pre-requisite for corrosion of steel


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Manufacturing / Refining Brings the

Material to a Higher Energy Level

The resistance to the oxidation (corrosion) varies fordifferent materials

When brought in contact with an electrolyte (aqueoussolution that conducts electricity) some metals easily

dissolve into ions. These are called Base metals

Noble metals have a very strong resistance to corrosion

and do not react to form ions unless exposed to veryaggressive chemicals

Oxidation of metals is a natural process


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Galvanic potentials in seawater


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Pre-requisites for corrosion

A Cathode:

The noble metal / alloy (or part of metal)

An Anode:

The less noble metal / alloy

An electrical connection between the two metals.Conducting electrical current (by electrons)

An electrolyte:

Conducting electrical current (by ions)

A galvanic cell consists of:


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A galvanic cell

In seawater, a calcareous deposit is formed on the steel surface

2 e

Zn = Zn + 2 e

Cathode:

Steel Anode:

ZincO2

O2 + H2 O + 2e = 2OH

2+ -

--

-

-


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How fast will a metal corrode ?

The speed of a corrosion process depends on:

The properties of the material

The surrounding environment


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Some environments are more

aggressive than others.

Steel will corrode at differentrates when exposed to

dissimilar environments, such

as seawater and fresh water

Seawater is more aggressivethan fresh water and will give

a higher corrosion velocityCP and corrosion: Corrosivity_liquid1


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Aluminium Zink Steel

Protective

Iron oxide

In a strong alcaline environment Aluminium and Zinc

will corrode rapidly, while steel will be passive

Corrosion speed differ when

exposed to the same environment

pH > 10


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What kind of information can be

obtained from the galvanic series ?

The difference in Galvanic Potential will tell which ofthe two materials that will corrode when connected

The metal with the more negative potential will corrode The metal with the more positive potential will be protected

Corrosion speed depends on the

difference in potential between the metals

The greater the potential difference, the greater the corrosion.Rule of thumb: A potential difference less than

50 mV will cause no additional corrosion


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Steel in SeawaterPotential versus Zinc & Ag/Ag Cl ref. electrodes

Rapid corrosion

General corrosion

Some corrosion

100% Cathodic protection

Overprotection

Possible coating damage

Increasing polarisation

Ag / Ag Cl Zinc

+ 0.50

- 0.25

+ 0.0

+ 0.25

- 0.55

- 1.30

-1.05

- 0.80

Potentials in volt

- 0.60

- 1.35

-1.10

- 0.85

Cu / CuSO4


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How to measure the corrosion

potential of a structure

+ -

Reference

electrode

Volt meter

Structure

Sea water


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How to measure the corrosion

potential of a structure

+ -


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Elements influencing the

corrosion speed of metals

Temperature

Salinity

Oxygen contentWater velocity

Acidity (See below)

Type of electrolyte ( e.g. cargo or chemicals)

Content of contaminants / pollution that

promotes corrosion

Micro-organisms.

Submerged materials


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The pH of the solution

will affect the corrosion speed

Steel

Corrosion increases at lower pH. (Acidic solutions,

pH 6 lower)

Strong alcaline solutions prevent corrosion(pH 10 higher)

Zinc and Aluminium

Slow corrosion at close to neutral solutions

Heavy corrosion in acidic and alcaline solutions


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Neutral

Acidic

Alcaline

pH-scale


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Parametres influencing the corrosion

speed. Atmospheric corrosion

Humidity

Temperature

Concentration of salts

Amount of air pollution,

including acid rain, soot and dust particles


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Atmospheric corrosion

corrosion rate depends on humidity

Relative Humidity, %

Corrosion rate

0 20 40 60 80 100


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CD 4911-0004CD 0026-001

General corrosion

General corrosion is uniform by natureStill, deep pits or uneven areas are found


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Uniform corrosion

A steel surface consists of noble and less noble areas This can be looked upon as small galvanic cells The anodic parts will corrode

Rust

++

++

-

-

-

+

-

-+

-

+ +-++


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Types of corrosion

1. Uniform corrosion2. Galvanic corrosion

3. Selective (preferential) corrosion

4. Pitting corrosion

5. Crevice corrosion

6. Micro biological corrosion (bacteria)

7. Corrosion fatigue

8. Stress corrosion cracking

9. Erosion corrosion

10. Cavitation

11. Stray current corrosion


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Apart from using paint and CP:

How to protect against corrosion ?

Good design

Avoid corrosion traps

Improved accessibility - maintenance Proper materials selection

Insulate between dissimilar materials

Change the surrounding environment

Remove water / humidity Apply metallic coatings

Use corrosion inhibitors (closed systems)

Corrosion protection can be achieved in many ways

Documents

01 Corrosion Grade1