corrosion

name	corrosion
description	Corrosion engineering — electrochemical corrosion, galvanic series, corrosion rate (mpy/mmpy), pitting, SCC, crevice, intergranular, erosion-corrosion, cathodic protection, coatings.
metadata	{"priority":7,"promptSignals":{"phrases":["corrosion","galvanic","pitting","SCC","cathodic protection","corrosion rate","rust","oxidation","electrochemical"],"minScore":3}}

Corrosion Engineering — Complete Skill

Electrochemical Basis

Oxidation-Reduction

Anode (oxidation): M → M^n+ + ne⁻ (metal dissolves) Cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻ (neutral/alkaline) OR 2H⁺ + 2e⁻ → H₂ (acid) Net reaction: Fe + ½O₂ + H₂O → Fe(OH)₂ → Fe₂O₃ (rust)

Mixed potential theory: corrosion potential E_corr where i_anodic = i_cathodic (open circuit) Corrosion current i_corr → corrosion rate via Faraday's law

Faraday's Law (Corrosion Rate)

Corrosion rate: CR = i_corr × M / (n × F × ρ) [m/s] CR [mm/yr] = 3.27 × i_corr [μA/cm²] × M [g/mol] / (n × ρ [g/cm³]) CR [mils/yr] = 0.129 × i_corr [μA/cm²] × EW / ρ [g/cm³] EW = equivalent weight = M/n

Corrosion severity: < 0.1 mm/yr: satisfactory for most applications 0.1-0.5 mm/yr: acceptable with corrosion allowance (add to wall thickness)

1 mm/yr: severe — redesign required

Galvanic Corrosion

Galvanic Series (in Seawater, Active → Noble)

Active (anodic, corrodes): Mg, Zn, Al alloys, Cd, Mild steel, Cast iron, Pb, Sn, Ni (active), Brasses, Copper, Bronze, Ni (passive), Monel, Stainless 316 (passive), Ti, Pt, Gold Noble (cathodic, protected) →

Galvanic corrosion rule: when two metals contact in electrolyte:

• Anode (less noble) corrodes accelerated
• Cathode (more noble) protected
• Larger potential difference → more galvanic attack
• Cathode/Anode area ratio matters: large cathode + small anode = severe attack

Practical limits: Safe (<50 mV potential difference): mix most copper alloys Marginal (50-250 mV): add protective coating to cathode Severe (>250 mV): isolate with gaskets, sleeve bolts, coat or avoid combination

Design rules:

• Larger anode, smaller cathode (reverse of what drives galvanic)
• Insulate dissimilar metals (nylon washers, dielectric gaskets)
• Paint cathodic metal (not anodic — holiday in anodic paint creates worse small anode)
• Use fasteners more noble than joined materials (SS bolt in carbon steel)

Corrosion Types

Uniform Corrosion

Entire surface corrodes uniformly Controlled by corrosion allowance in design; predictable Treatment: coatings, inhibitors, cathodic protection

Pitting Corrosion

Localized, deep pits; surface largely unattacked Most dangerous: hard to detect, stress concentrators Pitting Resistance Equivalent (PREN): PREN = %Cr + 3.3(%Mo) + 16(%N) PREN > 40: suitable for seawater (super-duplex, grade 6Mo) PREN > 25: suitable for industrial chloride environments 316L: PREN ≈ 25; 2205 duplex: PREN ≈ 35; 254 SMO: PREN ≈ 43

Stress Corrosion Cracking (SCC)

Requires: susceptible material + tensile stress + specific corrosive environment Simultaneous — any one absent → no SCC

Common SCC systems: Austenitic SS + chlorides (>150 ppm Cl⁻) + T>60°C → SCC High-strength steel + H₂O (hydrogen embrittlement) Cu alloys + ammonia (season cracking) Al alloys + Cl⁻ (T>80°C) Ti alloys + methanol, red fuming nitric acid

Prevention: choose resistant material, reduce stress (PWHT), coating, cathodic protection, control environment (lower Cl⁻, pH, temperature) SCC threshold: K_ISCC < K_IC (SCC propagates below fracture toughness)

Crevice Corrosion

In occluded zones: gaskets, under deposits, in threads, lap joints Mechanism: O₂ depleted → local acidification → accelerated attack Prevention: eliminate crevices (full penetration welds), use alloys with high PREN, add TIG dress to welds

Intergranular Corrosion (IGC)

Sensitization: 304/316 SS heated to 450-850°C → Cr₂₃C₆ precipitates at grain boundaries → Cr-depleted zones → susceptible Prevention: low-C grades (304L, 316L: C<0.03%), stabilized grades (347 with Nb), solution anneal

Erosion-Corrosion

High-velocity flow + corrosion → combined attack exceeds either alone Critical velocity: below threshold, protective film; above threshold, film removed → active surface Carbon steel pipe: limit flow velocity to 1.5 m/s (water) Cu pipe: limit 1.5 m/s (water), 1.0 m/s (hot water, higher temp)

Microbiologically Influenced Corrosion (MIC)

Bacteria-assisted: sulfate-reducing bacteria (SRB) produce H₂S → attack SS, carbon steel Indicators: black deposits, sulfide smell, localized pits Treatment: biocides, pigging, flow maintenance (>0.3 m/s)

Cathodic Protection (CP)

Impressed Current (ICCP)

External DC power supply → current to structure → structure becomes cathode Anode: graphite, high-silicon cast iron, MMO (mixed metal oxide) Protection criterion: E ≤ -850 mV vs. Cu/CuSO₄ (carbon steel in soil/seawater) Current density: 15-50 mA/m² (bare steel, seawater); 1-5 mA/m² (coated steel) Applications: underground pipelines, ship hulls, storage tanks, offshore platforms

Sacrificial Anode (SACP)

Connect base metal to more active (less noble) metal → base metal = cathode = protected Anodes: Zinc (seawater, soil), Aluminum (seawater, high electrolyte), Magnesium (soil, freshwater) Protection potential: same -850 mV CSE criterion Current output: Zn: 780 Ah/kg; Al: 2500 Ah/kg; Mg: 1100 Ah/kg

CP + Coating Synergy

Coating reduces bare metal area → reduces CP current requirement "Holiday" test: detect coating defects, then CP protects holiday areas CP without coating: very high current requirement; coating without CP: coating lifetime limits protection

Corrosion Inhibitors

Anodic inhibitors: form passive film on anode (chromate, nitrite, molybdate) Cathodic inhibitors: slow cathodic reaction (Ca²⁺, Zn²⁺, polyphosphates) Mixed inhibitors: organic films on both (amines, TOFA-based) Concentration: 50-5000 ppm typically; threshold effect (below threshold: accelerates corrosion)

Material Selection for Corrosion

Environment	Recommended Material
Freshwater	Carbon steel (with allowance), HDPE
Seawater	316L SS (low velocity), 2205 duplex (moderate), 6Mo SS/Ti (severe)
HCl (dilute)	Hastelloy C276, Rubber-lined CS
H₂SO₄ (dilute)	Hastelloy B, Alloy 20
H₂SO₄ (concentrated)	Cast iron, AISI 316L (>70%)
NaOH	Nickel alloys, carbon steel (avoid SCC of SS)
Chlorinated solvents	PVDF, PTFE, Carbon steel (dry)
High-temp oxidation	Inconel 601, 309/310 SS, FeCrAl alloys

Output

Provide: corrosion rate CR [mm/yr], type of corrosion (uniform/pitting/SCC/crevice), PREN for SS selection, galvanic compatibility assessment, CP current density required [mA/m²], coating + CP recommendation.