AMPP Corrosion Technologist cheat sheet

Criteria for buried steel: -850 mV vs Cu/CuSO4 with CP applied (IR-free), or 100 mV of polarization shift from native potential.

Instant-off (IR-free) reading removes voltage drop; measured value must still meet the -850 mV criterion.

Sacrificial anodes (Mg, Zn, Al) suit low-resistivity, low-current-demand structures; impressed current suits high demand and large structures.

Anode output by Ohm's law: I = (E_driving - E_structure) / R_circuit, where R_circuit includes anode-to-earth, structure-to-earth, and cable resistance.

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

Structure-to-electrolyte potential is measured with a high-input-impedance voltmeter and a reference electrode (Cu/CuSO4 for soil).

Stray-current corrosion occurs where current discharges from a structure; CP interference bonds drain current to the affected structure.

Overprotection risks coating disbondment from hydrogen evolution and embrittlement of high-strength steels; monitor potentials more negative than about -1200 mV.

Coating protects by barrier, inhibitive pigments, and galvanic (sacrificial) action; most systems combine barrier with one other mechanism.

Surface preparation per SSPC-SP standards: SP-10 near-white and SP-5 white blast give the best adhesion; SP-2 hand tool is minimal.

Anchor profile (typically 1.5-4 mils) improves mechanical adhesion; excessive profile can cause thin spots and rust-through.

Dew point rule: surface temperature must be at least 3 C (5 F) above dew point and rising during application.

Dry film thickness (DFT) relates to wet film thickness by DFT = WFT x volume solids fraction.

Holidays are discontinuities detected by high-voltage spark testing; low-voltage wet-sponge testing is used on thin films.

Cathodic disbondment occurs where CP current at a coating defect drives alkali formation and lifts the coating around the holiday.

Epoxy, polyurethane, and zinc-rich primers serve distinct roles: zinc-rich for galvanic protection, epoxy for immersion barrier, polyurethane for UV-resistant topcoat.

Galvanic series ranks alloys by corrosion potential in seawater; couple dissimilar metals only when the potential difference is small or the anode area is large relative to cathode.

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

Stainless steels rely on a passive chromium-oxide film (about 10.5% Cr minimum); chlorides and low oxygen break passivity, causing pitting.

Carbon steel suffers uniform corrosion in neutral aerated water; the rate is limited by oxygen diffusion to the surface.

Sulfide stress cracking affects high-strength steels in wet H2S; hardness limits (e.g., NACE MR0175/ISO 15156) control susceptibility.

Copper alloys resist seawater but are susceptible to erosion-corrosion at high flow and to ammonia-induced SCC in some brasses.

Duplex stainless steels combine austenite and ferrite for higher strength and chloride stress-corrosion resistance than 300-series austenitics.

Crevice corrosion initiates where oxygen is depleted (gaskets, lap joints); design to avoid tight crevices or specify higher-alloy material.

CO2 corrosion (sweet corrosion) of carbon steel depends on partial pressure of CO2, temperature, pH, and flow; protective iron carbonate scale can form at higher temperature and pH.

H2S corrosion (sour service) forms iron sulfide films; risk includes pitting and sulfide stress cracking of susceptible steels.

Erosion-corrosion increases with flow velocity above a critical threshold; elbows, tees, and reducers are high-risk locations.

Oxygen corrosion in closed or wet systems is controlled by deaeration and oxygen scavengers; even ppm levels cause pitting in boilers.

Microbially influenced corrosion (MIC) is associated with sulfate-reducing bacteria producing H2S; sessile populations on surfaces matter more than planktonic counts.

Water wetting is a prerequisite for internal aqueous corrosion; keeping water out of the line (below critical water cut) prevents corrosion.

Corrosion coupons and electrical resistance/linear polarization probes monitor internal rates; coupon location must match the flow regime of concern.

Galvanic corrosion internally occurs at dissimilar-metal joints (e.g., carbon steel to stainless); isolate with insulating kits.

Corrosion inhibitors include anodic (passivating), cathodic, and mixed types; anodic inhibitors can worsen localized attack if underdosed.

Filming amine inhibitors form a hydrophobic barrier on steel in pipelines; continuous injection maintains the film.

Scale inhibitors (phosphonates, polymers) work by threshold inhibition, crystal modification, and dispersion rather than stoichiometric chelation.

Oxygen scavengers: sodium sulfite for boilers, hydrazine/carbohydrazide for high-pressure systems; sulfite adds dissolved solids.

Biocides (oxidizing like chlorine, or non-oxidizing like glutaraldehyde) control MIC and biofouling; dosing must account for demand and contact time.

pH control is a primary corrosion lever; for carbon steel in CO2 service, raising pH favors protective iron carbonate scale.

Residual monitoring (e.g., inhibitor residual, scavenger residual) confirms treatment is present; it does not by itself confirm corrosion control.

Water treatment program selection balances corrosion, scale, and microbiological control; over-treating one can worsen another.