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Home , Cathodic protection, Galvanic series

4d.4d. CorrosionCorrosion

from L.H. VanVlack, Elements of Materials Science & Engineering p.154 of notes CorrosionCorrosion -- DefinitionDefinition

• is a form of surface deterioration of metallic materials through electrochemical means

• Like any reaction, corrosion is driven by the reduction in Gibbs free energy

• Many systems ranging over a number of industries are prone to corrosion. CostCost ofof CorrosionCorrosion

Total Cost: 275.7 billion/yr

(From Shaw and Kelly, Electrochemical Society Interface, 2006) ElectrochemicalElectrochemical ReactionsReactions

Metal atoms characteristically lose or give up electrons – Oxidation Reaction (Eq. 1) --- also known as Anodic Reactions; (M Mn+ + n e-) is the oxidation site

These electrons are transferred to another chemical species – Reduction Reaction (Eq.2) – Cathodic Reactions – is the reduction site

Example : many undergo corrosion in acid solutions with high H+ + ion concentration leading to reduction of H to evolve into H2 gas + - 2H + 2e H2 CorrosionCorrosion FundamentalsFundamentals •

Anode: (M Mn+ + n e-) (oxidation reaction) Cathode: (N + ne- Nn-) (reduction reaction) Electrolyte ions (Mn+) dissolve in the electrolyte Or combine with nonmetallic ions to form a surface deposit These 2 reactions must take place for corrosion to occur CorrosionCorrosion

Corrosion will stop if a) the electrical connection is interrupted, or b) the cathode reactants are depleted, or c) the anode products are saturated. AnodicAnodic ReactionsReactions

• Reactions 13-5.1Zn and 13-5.1Cu are anodic reactions releasing electrons • to an electric potential (but cannot be measured in isolation) Zn Cu voltage difference can be measured (1 molar solutions at 25C) • Zn becomes anode and Cu cathode (Fig. 13-5.2) Zn gets corroded & Cu gets plated 1 M solution is obtained by adding 1 mole per liter CathodicCathodic ReactionsReactions

•Apart from the electroplating reaction (Eq.13-5.2b), the following reactions are cathodic in nature:

+ a) evolution: 2H + 2e H2

- b) Hydroxyl formation: O2 + 2 H2O + 4e 4 (OH)

+ c) Water Formation: O2 + 4 H + 4e 2H2O

Half-cell reactions & Hydrogen reaction (a) is taken as reference StandardStandard EmfEmf SeriesSeries • The standard hydrogen reference half-cell (using Pt ) in a 1M solution of H+ ions

• The electromotive force (emf) series (Table 13-5.1) is generated by coupling to the standard hydrogen electrode. ElectrochemicalElectrochemical SeriesSeries c a t h ** o d 1 molar solution, 25oC i Potential varies with c concentration C & T

Nernst Equation: o E = EM + (k T / n) (ln C) n=no of electrons released/atom A n Table 13-5.1 o d i c GalvanicGalvanic SeriesSeries (Table 13-6.2)

• is a more practical chart showing relative tendency to corrosion in a particular environment (such as sea water)

• at the end of the series is the most active, and 18/8 stainless (passive condition) is on the top of the list. (Table 13-6.2)

• Ti and Ni which are more active in the electrochemical series are near the noble end of this galvanic series, by virtue of their ability to form passive films.

CorrosionCorrosion CouplesCouples

• Composition cells (e.g. galvanized steel / Zn-coated)

• Stress cells (involving dislocations, grain boundaries, and highly stressed regions)

• Concentration cells (arise from the difference in electrolyte composition) CompositionComposition CellsCells (Zn(Zn coatingcoating onon Steel)Steel) • A composition cell is established between two different metals or phases of different composition Fig. 13-5.4 • Sacrificial Anode: A metal more active on the electrochemical series is used to reduce corrosion of a less active one CompositionComposition CellsCells (Sn(Sn coatingcoating onon Steel)Steel) • coating on steel acts as merely a mechanical barrier preventing environment coming in contact with the steel plate underneath

• If a scratch on the surface exposing steel underneath is made, corrosion will start.

• Other examples: a) steel screws in marine Fig. 13-5.5 hardware b) steel pipe connected to plumbing

MicroscopicMicroscopic CompositionComposition CellCell (e.g.(e.g. )steels)

Any two-phase is more subject to corrosion than a single phase alloy (steels) (Fig. 13-5.6 on Al-Si) Microscopic Composition Cells (Grain boundary corrosion)

Fig. 13-5.9 GBs serve as anode due to higher energy of GB atoms Stress Cells • Stress cells do not require compositional difference in or concentration difference in the electrolyte; rather it can occur due to differences in dislocation densities, grain boundaries, and highly stressed regions.

• Presence of cells can significantly accelerate the corrosion rate (such as, stress corrosion cracking, corrosion fatigue). ConcentrationConcentration CellsCells

• Concentration cells arise from difference in electrolyte compositions.

Fig. 13-5.11 • Concentration cell aggravates corrosion where the electrolyte concentration is dilute – Crevice corrosion ConcentrationConcentration CellsCells –– OxidationOxidation TypeType

Of wider importance are oxidation- type concentration cells.

The oxidation cell aggravates corrosion where the concentration is low. Example: Crevice corrosion (Hydroxyl reaction at the cathode requires O – supplied by areas devoid of O – anode – see Fig) See also Fig. 13-6.4

This may often lead to a self‐aggravating situation, culminating into pitting corrosion. Intergranular Corrosion

Sensitization occurs upon arc welding of austenitic stainless steels and may lead to intergranular cracking

Cr23C6 Carbides Corrosion Protection

• Passive Surface Films:

a) Aluminum: Protective Al2O3 film is formed - anodizing process

b) : Cr2O3 film is formed in oxidizing conditions ()

c) Inhibitors: Chromate or highly oxidized ions are adsorbed onto the metal surface Corrosion Protection • Galvanic Protection:

a) sacrificial anode

b) impressed current method Stress Corrosion Cracking • Stress corrosion cracking (SCC) refers to the cracking of a material under static load by the combined action of a stress and a corrosive environment • Examples: , caustic embrittlement, season cracking etc. Brittle cracks in an otherwise ductile metal Intergranular Stress Corrosion Cracking (IGSCC) GeneralGeneral CharacteristicsCharacteristics ofof SCCSCC

• SCC is found in alloys and not in pure metals. However, there is serious uncertainty as to what level of purity can be classified as ‘pure’! • SCC occurs only in a specific environment for a given alloy. Some examples of environment are given below:

Alloy Environment - -1 Mild Steel Conc. NO3 , OH

High Strength Steel H2O Stainless Steel Cl- Aluminum Alloys NaCl solutions

Zr Alloys I2, Cd (PCI) GeneralGeneral CharacteristicsCharacteristics ofof SCCSCC • The presence of a tensile component of stress is necessary: may be externally applied or residual stress • The cracks in SCC move on a very narrrow front. In hydrogen cracking, the cracks are known as hair-line cracks (often difficult to see). If cracking is due to anodic dissolution, then extremely localized corrosion may take place. • The crack path is often intergranular. • Many of the alloys which are prone to SCC form passive films (not true always). • Cracking consists of two stages: (i) initiation and (ii) propagation. alloys are immune to crack initiation in chloride solutions. But when the alloys are pre-cracked, crack propagation is found to occur. SCCSCC –– EffectEffect onon PropertiesProperties

σ

σc air

σSCC environment

tr SCCSCC –– EffectEffect onon PropertiesProperties

(K(KIC))

Fig. 13-6.8 SCCSCC –– EffectEffect onon PropertiesProperties (Endurance(Endurance Limit)Limit)

air Endurance Limit

Environment SCCSCC –– EffectEffect onon PropertiesProperties

((ΔΔKKth)) CorrosionCorrosion TypesTypes -- SummarySummary • Pitting Corrosion • Crevice Corrosion • Intergranular Corrosion • Stress Corrosion • Corrosion Fatigue • Fretting Corrosion • Erosion-Corrosion; and many more

Neutron irradiation enhances corrosion SummarySummary

• Corrosion in metallic materials occurs through electrochemical means. • Electrochemical/Galvanic Series determines the relative activity of a metal in a corrosion cell • Corrosion cells can be of different types: composition cells, concentration cells and stress cells. • Various types of corrosion such as crevice corrosion, pitting corrosion, stress corrosion etc. may occur depending on various conditions. • Corrosion Protection can be achieved through passive film formation, painting, through sacrificial anode and impressed current method are discussed.