Galvanic corrosion. Åpen krets: ΔV = E korr (M) - E korr (N) > 0. e - Lukket krets (galvanisk kobling): E M = E N = E kobling

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1 Korrosjonstyper Generell (uniform) korrosjon Galvanisk korrosjon Punktkorrosjon (groptæring) Spaltkorrosjon Selektiv korrosjon Interkrystallinsk korrosjon Erosjonskorrosjon Spenningskorrosjon Filiform korrosjon

2 Uniform corrosion

3 Galvanic corrosion Åpen krets: ΔV = E korr (M) - E korr (N) > 0 e - Simple galvanic cell Lukket krets (galvanisk kobling): E M = E N = E kobling Positiv galvanisk strøm fra N til M I korr (N) øker I korr (M) avtar

4 Galvanic corrosion Effect of cathode/anode area ratio: i a A a = i c A c i a = i c (A c /A a ) Coating to minimize galvanic corrosion => Always coat the cathode

5 Galvanic corrosion high conductivity solution væske med høy ledningsevne Steel bolt Al sheet low væske conductivity med lav ledningsevne solution Under atmospheric conditions (with thin liquid film on surface): galvanic corrosion located near contact points cathode/anode area ratio not so important

6 Galvanic corrosion active noble Practical galvanic series in 1 M NaCl solution Influence of alloying elements (in solid solution) on potential of aluminium ΔV = driving force for galvanic corrosion

7 Galvanic corrosion

8 Punktkorrosjon (groptæring, pitting) Nødvendige forutsetninger: Oksidfilm (passiv tilstand) Nærvær av aggressive ioner, vanligvis klorid E > E c (kritisk pittingpotensial) Ekstern eller intern elektrisk drivende kraft (galvanisk kobling; intermetalliske partikler i metallet) Pittingkorrosjon - rustfritt stål

9 Pitting potential (V vs SCE) Pitting potential (V vs SCE) Critical pitting potential Factors affecting the critical pitting potential of aluminium: Temperature Chloride concentration Alloy composition (elements in solid solution) -0,6-0,7-0,8-0,5-0,6-0,9-1 -0,7-1, Temperature ( C) -0,8 0,001 0,01 0, Chloride concentration (mol/l)

10 Pitting corrosion of Al Fe (and Si) impurities in commercial aluminium alloys => Intermetallic Al-Fe-Si particles Pit initiation occurs at particles Most intermetallic phases are nobler than the Al matrix => preferred sites for the cathode reaction during corrosion SEM image of a rolled aluminium sheet surface (AA8006)

11 Pitting corrosion of aluminium Anode reaction: Al = Al e - Hydrolysis: Al H 2 O = Al(OH) 3 + 3H + => Acidic (chloride enriched) environment in anodic sites Cathode reactions: O H 2 O + 4 e - = 4 OH - 2 H e - = H 2 (g) => Build-up of alkaline diffusion layer at cathodic sites

12 Spaltkorrosjon (crevice corrosion) Oxygen in crevice is consumed => cathode reaction occurs on the outside Metal ion (M n+ ) concentration in crevice increasing Chloride ions migrate into crevice to maintain electroneutrality Aggressive conditions in crevice => low ph, high [Cl - ] Fe 3+ Low ph Cathode reaction Cl - O 2 + 2H 2 O + 4e - = 4OH - e - Fe Reactions in crevice: Fe = Fe e - Fe H 2 O = Fe(OH) 3 + 3H + => acidification

13 Selektiv korrosjon (selective leaching) Preferred corrosion of one element/constituent of a solid solution alloy; e.g., Zn from brass (Cu-Zn) CuZn alloy (brass) (a) Large potential difference between Cu (noble) and Zn (active) in the galvanic series Porous Cu (b)

14 Interkrystallinsk korrosjon Korrosjon av korngrenser Rustfrie Cr/Ni stål: Utfelling av krom-karbid partikler på k.g. ved varmebehandling eller sveising Utarming av Cr på begge sider av korngrensene Tiltak: Bruke stålkvaliteter med lavt C- innhold (< 0.02%) Stabilisering ved tilsats av Nb, Ti Varmebehandling etter sveising

15 Interkrystallinsk korrosjon av Al-legering Accelerated corrosion test Grain boundary corrosion in AlMgSi(Cu) alloy is very dependent on heat treatment. What is the underlying mechanism? G. Svenningsen, 2005

16 Interkrystallinsk korrosjon av AlMgSi(Cu) legering - Utfelling av Cu på korngrenser M. Hurlen Larsen, 2010

17 Erosjonskorrosjon Fler-fase strømning faste partikler gassbobler => Mekanisk nedbryting av oksidfilm To typer erosjonskorrosjon Støterosjon ("impingement") Breakdown of passivating layer by erosion (pipe elbows) Kavitasjon:

18 Spenningskorrosjon (stress corrosion) Sustained tensile stress Specific corrosive environment Stress corrosion Susceptible material

19 Alloy - environment systems exhibiting SCC Rivet holes

20 Stress corrosion cracking (SCC) SCC likely to occur at a transition between corrosion and passivity Initiation of cracks at stress raisers - surface irregularities, defects, inclusions Can be transgranular or intergranular Many propagation mechanisms proposed none generally applicable - adsorption of hydrogen atoms at crack tip => hydrogen embrittlement Rivet holes

21 Hydrogen embrittlement Martensitic steel Fractured steel surface (SEM) Brittle fracture due to diffusion of atomic hydrogen into metal. Sources of hydrogen: Cathodic protection Welding Surface treatment (pickling of steel) H + + e - => H ads = > ½ H 2 (g) H 2 S: "Poison" (promotes HE)

22 Filiform corrosion (FFC) Occurs under coating on painted metal surfaces Initiate at coating defects Superficial, mainly aesthetic significance Occurs by exposure to humid marine/industrial atmosphere

23 FFC growth by successive pitting (slow) Polymer coating Corrosion products (Al oxide) Aluminium Repeated pitting events accompanied by mechanical lifting of organic coating by voluminous corrosion products H. Leth-Olsen, 1996

24 FFC growth by anodic undermining (rapid) Cold rolled Cold rolled + heat treated

25 Korrosjonsbeskyttelse Materialvalg Design/konstruksjonsutforming Katodisk beskyttelse Overflatebehandling/belegg organiske, metalliske, keramiske Korrosjonsinhibitorer

26 Design/konstruksjonsutforming Continuous welding better (avoid crevice) Poor Better

27 Cathodic protection

28 Cathodic protection sacrificial anodes

29 Cathodic protection by metallic coatings Galvanized steel Cladding of Al aircraft alloy zinc Zn 2+ zinc 2e - 2e - Carbon steel Al-Zn alloy Al-Cu-Mg alloy (AA2024)

30 Coating Corrosion protection Decorative purposes Bibliotheca Alexandria in Egypt Powder coated aluminium profiles Burj al Arab in Dubai Powder coated aluminium window frames

31 Surface pre-treatment Pre-treatment before application of organic coating is essential Degreasing Etching Conversion coating Purpose of conversion coating Provide good bonding to organic coating

32 Chromate conversion coating Thinning of native Al-oxide: Al 2 O HF = 2 AlF 3 (soluble) + 3 H 2 O Oxidation: 2 Al = 2 Al e - Reduction: Cr 2 O H e - = 2 Cr(OH) 3 + H 2 O 2 Al + Cr 2 O H + = 2 Al Cr(OH) 3 + H 2 O Acceleration of conversion process by addition of ferricyanide: a Al + Fe(CN) 6 3- Cr 6+ + Fe(CN) 6 4-1µm Al 3+ + Fe(CN) 6 4- Cr 3+ + Fe(CN) 6 3- a-al(fe,mn)si Pre-treatment for Al alloys, Zn-coated steel, Mg alloys Highly effective but toxic, classified as human carcinogen Stringent disposal limits b Chromate-free alternatives required 1 µm

33 Anodic oxides (anodising) Al-, Ti- and Mg alloys Thick coatings (~ 20 μm) Corrosion protection Decorative surfaces (may be coloured) Abrasion resistance Thin coatings (< 1 μm) Base for paint and adhesives

34 Anodising of Al Porous oxide film Electrolyte 15% H 2 SO 4 Temperature 20ºC Current mode DC Current density 1.5 A/dm 2 Voltage 15 V Time 30 min Barrier layer Aluminium 100nm

35 Corrosion inhibitors Added in low concentrations to corrosive environment Closed systems Heat exchangers (intermittent treatment) Oil wells, pipelines (continuous treatment) Classification according to mechanism ph controlling [Fe: høy ph; Al: nøytral ph] Oxygen scavengers (e.g. sulphite) [sulfitt + oksygen => sulfat] Protective films [fosfater => Fe-fosfat sjikt] Anodic (blocking of anodic sites) Cathodic (suppresses cathodic reaction)

36 Oxidation (dry) M + ½ O 2 MO Ox: M M e - Red: ½ O 2 + 2e - O 2- Pilling-Bedworth ratio R < 1 Porous non-protective R ~ 1 Protective, adherent R > 2-3 Non-adherent (high compressive stresses, cracking) P B A O M A M O Other factors affecting the oxidation resistance: Thermal expansion of oxide vs. metal High temperature plasticity of oxide

37

38 Thermal oxidation of aluminium TEM cross-sections of pure aluminium after heat treatment for 1 h at 600 C Crystallization of oxide when T > 450ºC Al-Mg alloys: Mg-rich oxide films form