Corrosion Study Guide

📖 Core Concepts Corrosion – gradual chemical/electrochemical deterioration of a metal, converting it to a more stable oxide, salt, etc. Electrochemical cell – metal surface contains anodic spots (metal → Mⁿ⁺ + ne⁻) and cathodic spots (O₂/H⁺ + e⁻ → H₂O). Electrons travel through the metal, ions travel through the electrolyte. Diffusion‑controlled – rate is limited by how fast reactants (O₂, H⁺, aggressive ions) reach the metal surface. Reducing surface activity (passivation, coating) → higher resistance. Uniform vs. Localized corrosion – uniform attacks the whole exposed area evenly; localized (pitting, crevice) concentrates in tiny zones, causing deep damage despite little overall loss. Galvanic series – ranking of metals in a given electrolyte by anodic tendency; higher ⇒ more active (anodic). Pairing a less‑noble metal with a more‑noble one creates a galvanic cell. Passivation – spontaneous formation of an ultrathin (≈10 nm) oxide film that blocks electron transfer; common on Al, stainless steel, Ti. --- 📌 Must Remember Corrosion reaction (in oxygenated water): $$\text{Anode: } \text{M} \rightarrow \text{M}^{n+} + n e^-$$ $$\text{Cathode: } \frac{1}{2}O2 + 2H^+ + 2e^- \rightarrow H2O$$ Weight‑loss corrosion rate: $$R = \frac{k\,W}{A\,\rho\,t}$$ \(W\) = weight loss, \(A\) = exposed area, \(\rho\) = density, \(t\) = time, \(k\) = constant Sacrificial anodes – Zn, Mg, Al are “high” on the galvanic series; they oxidize preferentially to protect the structure. Pitting initiation – breakdown of passive film → oxygen‑starved, acidic pit interior → autocatalytic metal dissolution. Crevice cell – limited electrolyte exchange creates differential aeration (oxygen‑rich outside, oxygen‑poor inside). Cathodic protection types – (1) Sacrificial anode CP (galvanic), (2) Impressed Current CP (external DC source, inert anodes). Anodic protection – applies a small anodic current to force a passivating metal (e.g., stainless steel) into its passive region, used in strong acids. Pitting Resistance Equivalent Number (PREN) – predicts stainless‑steel pitting resistance (not detailed in outline, but mentioned as a high‑yield term). --- 🔄 Key Processes General electrochemical corrosion Anodic dissolution of metal atoms. Electron flow through metal to cathodic site. Cathodic reduction of O₂ (or H⁺) in electrolyte. Ionic migration to maintain charge balance. Galvanic corrosion Two dissimilar metals contact in electrolyte. More active metal becomes anode → rapid dissolution. More noble metal becomes cathode → protected. Rate ↑ with larger anode‑to‑cathode area ratio. Pitting corrosion Passive film locally damaged (mechanical, chemical, chloride attack). Exposed area becomes anodic; surrounding metal stays cathodic. Pit interior becomes O₂‑starved, acidic → accelerates dissolution. Pit deepens until film re‑forms or metal fails. Crevice corrosion Crevice forms (gasket, bolt‑hole, etc.). Electrolyte inside limited → O₂ depletion, pH drop. Differential aeration cell drives anodic dissolution inside crevice. Cathodic protection (sacrificial) Attach Zn/Mg/Al anode to structure. Anode oxidizes, supplying electrons to structure → structure becomes cathode → corrosion stops. Impressed Current CP Connect inert anodes to DC power source. Apply current that forces structure to cathodic potential. Anodic protection Apply small anodic current to passivating metal. Metal stays in passive region (low corrosion current). --- 🔍 Key Comparisons Uniform corrosion vs. Localized corrosion Uniform: even material loss, predictable rate. Localized: pits/crevices, high stress concentration, rapid failure. Galvanic corrosion vs. Pitting corrosion Galvanic: requires two dissimilar metals, whole anode corrodes. Pitting: occurs on a single metal with a passive film, confined to tiny spots. Sacrificial anode CP vs. Impressed Current CP Sacrificial: simple, self‑powered, limited current, good for small structures. ICCP: controllable current, suitable for large pipelines, tanks, offshore platforms. Passivation vs. Coating Passivation: thin, self‑forming oxide; relies on chemistry, can be damaged by chlorides. Coating: physical barrier (paint, plating); protects until breach, then can trap moisture. High‑temperature corrosion vs. Low‑temperature corrosion High‑temp: oxidation with O₂/S, formation of protective glaze; rate often diffusion‑controlled through solid oxide. Low‑temp: mostly aqueous electrochemical processes, diffusion of O₂/H⁺ in liquid. --- ⚠️ Common Misunderstandings “Coatings eliminate corrosion forever.” Coatings only isolate metal; breaches create under‑film corrosion, sometimes worse. “All stainless steel is immune to corrosion.” Stainless steels can pit, crevice, and stress‑corrode, especially in chloride environments. “Larger cathodic area always reduces corrosion rate.” In galvanic pairs, a large cathode can accelerate anode dissolution (higher anode‑to‑cathode area ratio). “Cathodic protection works for any metal.” CP is ineffective for metals that are already passive (e.g., some Al alloys) unless a proper anode is chosen. “Weight‑loss method measures instantaneous corrosion rate.” It gives an average rate over the exposure period, not a momentary value. --- 🧠 Mental Models / Intuition Cell diagram – Imagine the metal surface as a tiny battery: anode (metal loss) → electrons → cathode (oxygen reduction). Pit as a “starved furnace.” – Inside a pit, O₂ is depleted, H⁺ builds up → metal dissolves faster, like a self‑heating reaction. Galvanic series ladder – Higher rungs = more eager to give up electrons; when two rungs touch, the higher one corrodes. Diffusion bottleneck – Think of corrosion as traffic: the slower the road (diffusion path), the slower the cars (reactants) arrive, controlling the overall speed. --- 🚩 Exceptions & Edge Cases High‑temperature oxide glaze can protect a metal (e.g., stainless steel at >800 °C). Quartz, B₂O₃, P₂O₅ glasses have negligible alkaline extractability → appear highly durable in ISO 719 test despite different chemistry. Anaerobic corrosion – occurs without O₂, often driven by sulfate‑reducing bacteria (MIC) producing H₂S. Hydrogen embrittlement – hydrogen uptake (from corrosion or cathodic processes) can reduce ductility, leading to cracking. --- 📍 When to Use Which Select sacrificial anode material → Use Zn, Mg, Al when the protected metal is less noble (e.g., steel) and the environment is seawater or soil. Choose CP method → Small buried pipelines → sacrificial; large offshore structures → ICCP with inert anodes. Apply passivation vs. coating → If environment is mildly aggressive and metal naturally forms a stable oxide (Al, Ti), rely on passivation; for highly aggressive or chloride‑rich media, add a protective coating. Pick corrosion‑resistant alloy → For chloride exposure, select low‑carbon or alloyed stainless steel (e.g., 321, 347) and verify PREN is adequate. Use weight‑loss vs. electrochemical monitoring → Weight‑loss for long‑term average rates; electrochemical techniques (e.g., polarization) for rapid screening. --- 👀 Patterns to Recognize Chloride presence → pit risk (especially on stainless steel, Al). Confined geometry (gasket, bolt hole) → crevice corrosion. Two dissimilar metals in electrolyte → galvanic couple. High temperature + sulfur compounds → sulfidation/high‑temp corrosion. Rapid weight loss + constant current → uniform corrosion; localized weight loss with deep pits → pitting. --- 🗂️ Exam Traps “The more noble the metal, the slower it corrodes in any situation.” False: In a galvanic pair, the noble metal becomes cathodic and may still corrode if the anode is absent (e.g., oxygen‑starved cathode). “All coatings provide the same protection.” Wrong: Active plating (Zn, Cd) offers sacrificial protection; inert paints merely isolate and can trap moisture. “Impressed current CP always eliminates corrosion.” Misleading: Incorrect current density or poor anode placement can cause accelerated corrosion or hydrogen embrittlement. “Weight‑loss rate is independent of specimen geometry.” Incorrect: Area term \(A\) is crucial; larger surface area reduces measured rate if weight loss is the same. “Passivation is permanent and unaffected by environment.” Trap: Aggressive ions (Cl⁻, F⁻), low pH, and high temperature can break down passive films, leading to pitting. ---