AMPP Corrosion Technician cheat sheet

Criterion for steel in soil or water: polarized potential of -850 mV versus a copper/copper-sulfate reference electrode (CSE), measured with the CP current applied and IR drop compensated.

Alternative criterion: 100 mV of cathodic polarization (depolarization from instant-off to native potential) confirms protection when the -850 mV criterion is not met.

Sacrificial anodes (galvanic) are chosen when soil resistivity is low (typically below about 5,000 ohm-cm) and current demand is small; impressed current is used for high resistivity or high current demand.

Anode output formula (impressed groundbed): I = driving voltage / total circuit resistance, where total resistance includes anode-to-earth, cable, and structure-to-earth terms.

Dwight formula for a single vertical anode: R = (rho / (2piL)) * (ln(4L/d) - 1), with rho in ohm-cm, L and d in cm.

Common anode materials: high-silicon cast iron and mixed-metal oxide (MMO) for impressed current; magnesium, zinc, and aluminum alloys for galvanic service.

Magnesium anodes suit soil and fresh water; zinc anodes suit seawater and low-resistivity environments; aluminum anodes are used mainly in seawater.

Stray-current corrosion is identified when a structure-to-soil potential shift is caused by an external current source; mitigation includes drainage bonds, shielding, or relocating the source.

CP interference testing compares structure-to-soil potentials with the foreign source on and off; a positive shift indicates current pickup, a negative shift indicates discharge (corrosion risk).

Coating performance depends on surface preparation, coating selection, application, and inspection; surface preparation is the single largest factor in long-term performance.

NACE/SSPC surface preparation standards: SSPC-SP 1 solvent cleaning, SP 2 hand tool, SP 3 power tool, SP 5/NACE 1 white metal, SP 6/NACE 3 commercial blast, SP 10/NACE 2 near-white blast.

Abrasive blast cleanliness grades per ISO 8501-1: Sa 1, Sa 2, Sa 2 1/2, and Sa 3, with Sa 2 1/2 the common specification for immersion service.

Anchor profile is specified by abrasive size and measured by replica tape or profilometer; typical profile for epoxy is 1.5 to 4 mils (38 to 100 micrometers).

Holiday (pinhole) detection uses low-voltage wet-sponge testing for thin films and high-voltage spark testing per film thickness; excessive voltage can damage sound coating.

Cathodic disbondment occurs when CP current generates alkalinity and hydrogen at coating defects; resistance improves with surface cleanliness and appropriate primer chemistry.

Coating resistance to CP current is quantified by the coating conductance (inverse of resistance); a well-coated structure typically shows high resistance and low current demand.

Coating failure modes include blistering (osmotic or gas), undercutting, chalking, cracking, and disbondment; each maps to a distinct root cause such as soluble salts or inadequate cure.

Soluble salt contamination (chlorides, sulfates) on the substrate must be removed and verified by conductivity testing before coating application.

Galvanic series ranks metals by corrosion potential in seawater: noble metals (graphite, platinum, stainless steel) at the top, active metals (magnesium, zinc, aluminum) at the bottom.

Galvanic corrosion risk increases with greater potential difference, larger cathode-to-anode area ratio, and higher conductivity of the electrolyte.

The cathodic member of a couple is protected; the anodic member corrodes faster. Avoid coupling a small anode to a large cathode.

Austenitic stainless steels (304, 316) are susceptible to chloride pitting and stress-corrosion cracking; 316 contains molybdenum for improved chloride resistance.

Carbon steel in neutral aerated water corrodes by oxygen depolarization; the rate is limited by oxygen diffusion to the cathode.

Pitting resistance equivalent number (PREN) = %Cr + 3.3*%Mo + 16*%N; higher PREN indicates better chloride pitting resistance.

Sensitization of austenitic stainless steel occurs when chromium carbides precipitate at grain boundaries (typically 425 to 815 C / 800 to 1500 F), depleting chromium and enabling intergranular attack.

Copper alloys resist seawater but are susceptible to erosion-corrosion and dezincification; admiralty and 90/10 Cu-Ni are common marine choices.

Hydrogen embrittlement affects high-strength steels and can be aggravated by overprotection (excessive cathodic current producing hydrogen at the surface).

Inhibitors are classified by mechanism: anodic (passivating), cathodic, and mixed; anodic inhibitors can cause severe localized attack if underdosed.

Anodic inhibitors include chromates, nitrites, molybdates, and phosphates; they raise the corrosion potential and promote passive film formation.

Cathodic inhibitors include zinc salts, polyphosphates, and amines; they reduce the cathodic reaction rate, often by precipitating a barrier film.

Adsorption-type inhibitors (amines, imidazolines, quaternary ammonium compounds) form a film on the metal surface and are widely used in oil and gas pipelines.

Dosing must be maintained above a critical concentration; underdosing anodic inhibitors can shift attack from general to pitting.

Scale and corrosion control in cooling water relies on pH, Langelier Saturation Index, and inhibitor choice; a positive LSI indicates scaling tendency, negative indicates corrosive tendency.

Oxygen scavengers (sulfite, hydrazine, DEHA) remove dissolved oxygen in boiler systems; sulfite is not used where feedwater temperature exceeds about 200 C.

Filming amines protect condensate return lines by forming a hydrophobic film on metal surfaces.

Biocides (oxidizing such as chlorine or non-oxidizing such as glutaraldehyde) control microbiologically influenced corrosion (MIC) in cooling and process water.

The corrosion cell requires an anode, a cathode, an electrolyte, and an electrical path; removing any element stops the corrosion reaction.

Anodic reaction for iron: Fe -> Fe2+ + 2e-. Cathodic reaction in acidic solution: 2H+ + 2e- -> H2; in neutral aerated water: O2 + 2H2O + 4e- -> 4OH-.

Faraday's law: mass loss = (I * t * M) / (n * F), where I is current, t is time, M is atomic mass, n is electrons, F is 96,485 C/mol.

Corrosion rate conversion: 1 microamp per square centimeter equals approximately 0.0116 mm/year for iron (using equivalent weight and density).

Corrosion rate (mpy) = 0.128 * (current density in microamps/cm2) * (equivalent weight / density in g/cm3).

Uniform corrosion is measured by weight loss or linear polarization; localized forms (pitting, crevice, galvanic, SCC) require inspection and specific test methods.

NACE MR0175 / ISO 15156 addresses sulfide stress cracking in oil and gas service; material selection depends on H2S partial pressure, pH, and chloride content.

Erosion-corrosion accelerates with flow velocity, entrained solids, and turbulence; it is often localized at elbows, tees, and downstream of orifices.

Crevice corrosion is driven by oxygen depletion and chloride concentration inside the crevice; it is mitigated by design (avoid tight gaps) and alloy selection.