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Basics of Corrosion

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Basics of Corrosion 4d.4d. CorrosionCorrosion from L.H. VanVlack, Elements of Materials Science & Engineering p.154 of notes CorrosionCorrosion -- DefinitionDefinition • Corrosion 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-) Anode is the oxidation site These electrons are transferred to another chemical species – Reduction Reaction (Eq.2) – Cathodic Reactions – Cathode is the reduction site Example : many metals 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 • Galvanic cell Anode: (M Mn+ + n e-) (oxidation reaction) Cathode: (N + ne- Nn-) (reduction reaction) Electrolyte Metal 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 • Lead 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) Hydrogen 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 electrode) 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 *Electrode Potential* 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) • Galvanic series is a more practical chart showing relative tendency to corrosion in a particular environment (such as sea water) • Magnesium at the end of the series is the most active, and 18/8 stainless steel (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) • Tin 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 brass marine Fig. 13-5.5 hardware b) steel pipe connected to copper plumbing MicroscopicMicroscopic CompositionComposition CellCell (e.g.(e.g. steels)steels) Any two-phase alloy 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 electrodes 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 oxygen 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) Stainless Steel: Cr2O3 film is formed in oxidizing conditions (passivation) 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: hydrogen embrittlement, 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. Titanium 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, cathodic protection through sacrificial anode and impressed current method are discussed..
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