Corrosion protection

1
Corrosion protection

• Jacek Banas

University of Science and Technology
(AGH-UST) Faculty of Foundry Engineering Departmen
t of Chemistry and Corrosion of Metals
2
Corrosion protection

• Corrosion resistant alloys
• Coatings
• Anodic and cathodic protection
• Inhibitors

3
CORROSION PREVENTION
Corrosion Resistant Alloys Metals
thermodynamically stable in aqueous environments
- noble metals Cu, Ag, Au, Pt, Pd etc. Pourbaix
diagrams allow to predict stability conditions
for pure metals in aqueous solutions (pH, redox
potential and concentration of soluble
species). Metals and alloys thermodynamically
unstable Corrosion resistance is connected with
the passivation of metal surface stainless
steels, nickel and nickel alloys, high silicon
cast iron, valve metals (Ti, Ta, Al), aluminium
alloys. Corrosion resistance is connected with
the formation of insoluble layer of corrosion
product low alloyed steels (weathering
steels), lead, zinc and zinc coatings, tin and
tin plates, copper and copper alloys.
4
CORROSION RESISTANT ALLOYS active, passive and
transpassive state
Anodic polarization curves of metal undergoing
corrosion and passivation
5
Alloying by addition of the inhibiting components
and components improving protective behavior of
surface layer
Some non metallic (P,N, Si) and metallic (Cr, Mo)
components of the alloy can accumulate on active
places of metal surface during corrosion process.
These elements react with the solvent and form
non soluble strongly adsorbed compounds on the
kinks, steps and structural defects. The blocking
of active areas of the alloy surface leads to the
decrease of corrosion rate. The adsorbed
intermediate product can influence kinetics of
growth of the layer of corrosion product and can
change their microstructure and morphology. The
reason of the better corrosion resistance of
weathering steels in atmosphere in compare to
ordinary carbon steels is the presence of small
amounts of Cr, Mo, Si, P. The above mentioned
elements change the microstructure of iron
oxides on the steel surface. The protective
properties of the films formed on weathering
steels are connected with the presence of
superparamagnetic goethite and maghemite in the
inner layer. The decrease of particle size of
goethite and maghemite increases protective
behavior of the surface layer. The presence of
silicon and phosphorus in the weathering steels
stimulates the formation of superparamagnetic
goethite and thus improves the corrosion
resistance of the alloys.
6
CORROSION RESISTANT ALLOYS
Schematic presentation of the factors improving
corrosion resistance of active alloys
7
Optimal composition of carbon steel resistant to
hydrogen embrittlement
8
Passive metals
Passivity promoters and dissolution moderators
according to the synergy between the energy of
the metal-metal bonds and heat of adsorption of
oxygen. P. Marcus Corr.Sci. 36, 2155 (1994)
9
Coatings

• Coatings of metallic, inorganic and organic
  materials can provide a satisfactory barrier
  between metal and its environment and can protect
  the metal surface against corrosion.
• The following kinds of anticorrosion coatings can
  be distinguished
• metallic coatings
• inorganic coatings
• organic coatings
• composit materials

Metallic coatings -cathodic (Ni, Cr, Cu and
copper alloys, noble metals - Au, Ag, Pt) These
coatings are composed from corrosion resistant
metals and in corrosive medium, are more noble
than underlying metal surface (are cathodic in
reference to the protected metal). The damage of
the layer creates the corrosion macro galvanic
cell with the anodic underlying metal surface and
cathodic metal coating. anodic (Zn, Cd,
Al) Composed from metals which show more negative
stationary potential in corrosive medium (are
anodic in reference to the underlying metal
surface). These coatings can prevent underlying
metal surface even by the damage or perforation
of the layer - cathodic protection.
10
Coatings
Inorganic coatings oxid films -
anodization conversion layers chromates phosphates
silica layers ( sol-gel method) enamels Organic
coatings anticorrosion painting polymer coatings
11
METALLIC COATINGS Methods of preparation elect
rodeposition cementation chemical
reduction metallizing - flame spraying cladding Ho
t dipping vapour deposition diffusion surface
modification (laser surface alloying, ion
implantation) Electrodeposition Zn2 2e ?
Zn alcaline or acid media cementation Fe Cu2
? Fe2 Cu Chemical reduction Ni2 H2P02-
H20 ? Ni 2H H2P03-
12
INORGANIC COATINGS
Oxide films anodization of aluminium Al
2H20 ? AlOOH 3H 3e oxidation of iron
(NaOH NaN03 NaN02 (140 - 145C) ) Fe ?
Na2Fe02 ? Na2Fe204 ? Fe2O3 mH20 Conversion
layers nMen mA-n ? MemAn mne chromate
layers, phosphate layers
13
Cathodic protection
Current i
pipe cathode
anode
14
Cathodic protection

• Structures that are commonly protected by
  cathodic protection
• are the exterior surfaces of
• pipelines
• ships hulls
• storage tank bases
• jetties and harbour structures
• steel sheet, tubular and foundation pilings
• offshore platforms, floating and sub sea
  structures
• Cathodic protection is also used to protect the
  internal surfaces of
• large diameter pipelines
• ships tanks (product and ballast)
• storage tanks (oil and water)
• water-circulating systems.

Cathodic protection can be achieved in two
ways - by the use of galvanic (sacrificial)
anodes, or - by impressed current.
15
Anodic protection
16
Anodic protection
17
Inhibitors
Adsorption of organic inhibitor onto a metal
surface in aqueous environment
18
Inhibitors
Physical Adsorption - is the result of
electrostatic attrative forces between inhibiting
organic ions or dipoles and the electrically
charged surface of metal. The surface charge can
be defined by the potential of the metal (Ecorr)
vs. its zero-charge potential (ZCP, Eq0). When
the difference F Ecorr - Eq0 is negative,
cation adsorption is favored. Adsorption of anion
is favored when F is positive. Chemisorption -
involves charge sharing or charge transfer from
the inhibiting molecule to the metal surface in
order to form a coordinating type of
bond. Inhibition efficiency of homologous series
of organic substances differing only in the
heteroatom PgtSegtSgtNgt0
19
Inhibitors

• anodic inhibitors
• phosphates
• silicate compounds

• Cathodic inhibitors
• poly-phosphates
• Ca(HCO3)2
• methylamino-phosphate

(c) mixed anodic and cathodic inhibitors amines
selenides
20
Inhibitors
Types of Corrosion Inhibitors Organic These
materials are characterized by high molecular
weight structures, incorporating nitrogen or
phosphorous groups. They are usually highly polar
molecules. Phosphate Esters Phosphonates Inor
ganic Salts of some metals and amphoteric
elements act as corrosion inhibitors. Quite often
these materials have tenacious filmforming or
passivation effects. In some instances, they
react with the metal surface. Chromate
Salts Zinc Salts Molybdate Compounds Phospha
tes Nitrite Salts Silicate Compounds
21
Control Questions

• Write down Butler-Volmer equation for reaction
• Fe Cl- FeCle
• 2. Define corrosion potential of metal.
• 3. Define polarisation resistance. Discuss its
  importance for corrosion rate monitoring.
• 4. Write down the mechanism of iron passivation
  in aqueous solutions.
• 5. Is passivation of metals possible in
  nonaqueous solutions? Explain its mechanism.
• 6. What is the reason of better corrosion
  resistance of weathering steels and low-alloyed
  steels in aqueous solutions?
• 7. Give the main principle of stainless steel
  chemical composition selection.
• 8. Describe the principles of charting Pourbaix
  diagrams.
• 9. Write down reactions of atmospheric corrosion
  of iron under country atmosphere (lack of air
  pollution).
• 10 Write down the mechanism of carbon steel
  corrosion in H2O-CO2 solutions.
• 11. Discuss the main reason of hydrogen corrosion
  of metals.
• 12. What is the noxious effect of sulphate
  reducing bacteria (SRB) on corrosion resistance
  of metals?
• 13. Discuss principles of cathodic corrosion
  protection.
• 14. Discuss principles of anodic corrosion
  protection.
• 15. What is the protective effect of zinc
  coatings
• 16. What are the conversion coatings? Give
  examples.