AMPP Corrosion Specialist cheat sheet

Electrochemical corrosion requires four elements: an anode where oxidation occurs, a cathode where reduction occurs, an electrolyte that conducts ions, and a metallic path for electron flow. Removing any one element stops the cell.

Faraday's law: m = MIt/(nF). A uniform current density of 1 microamp per square centimetre dissolves iron at about 11.6 micrometres per year; 1 mpy equals 0.0254 mm/yr.

Nernst equation at 25 degrees C: E = E-standard minus (0.0592/n) times log Q. It converts standard potentials to actual half-cell potentials at the prevailing ion activity.

A Pourbaix diagram maps thermodynamic stability against potential and pH into immunity, passivity, and corrosion regions; it indicates tendency only and gives no kinetic rate information.

Galvanic corrosion severity rises with the potential difference between the coupled metals, with electrolyte conductivity, and with a large cathode-to-anode area ratio.

Differential aeration: the oxygen-depleted region becomes anodic and corrodes while oxygen-rich regions become cathodic. This drives crevice attack and corrosion under deposits.

Pitting requires local breakdown of a passive film, is promoted by chlorides, and is characterised by a pitting potential; pits penetrate deeply while total weight loss stays small.

Stress corrosion cracking requires a susceptible alloy, tensile stress (applied or residual), and a specific environment simultaneously, for example chloride for 304 or 316 stainless steel.

Corrosion rate classes in mm/yr: below 0.025 excellent, 0.025 to 0.13 good, 0.13 to 0.5 fair, above 0.5 poor; convert mpy to mm/yr by multiplying by 0.0254.

Match material to environment: carbon steel with coating plus cathodic protection for immersion, austenitic stainless for atmospheric and mild chloride service, duplex or superduplex for chloride-bearing process, nickel alloys for hot acids.

PREN = percent Cr + 3.3 times percent Mo + 16 times percent N. Type 316L is about 26, 2205 duplex about 35, superduplex about 40 and above; seawater service generally needs PREN above 40.

Avoid a small anode joined to a large cathode; where dissimilar metals must be coupled, insulate the joint or coat the cathode and keep the anodic area as large as practicable.

Design for drainage and cleaning: eliminate liquid traps, specify full-penetration butt welds instead of lap or fillet welds, and provide slopes and drain nozzles to prevent stagnant pools.

A corrosion allowance applies only where uniform thinning is the governing mechanism; it does not protect against pitting, stress corrosion cracking, or localized attack.

NACE MR0175 / ISO 15156 governs materials for sour service, invoked when H2S partial pressure exceeds 0.00035 MPa (0.05 psi) and limiting carbon steel to 22 HRC maximum hardness.

Velocity limits: carbon steel in seawater is prone to erosion-corrosion above roughly 1.5 m/s, copper-nickel and superduplex tolerate higher velocities, and dead legs promote under-deposit attack.

Cathodic protection current density for bare steel in seawater is about 100 to 110 mA per square metre and for well-coated steel about 1 to 25 mA per square metre, so coating quality controls anode mass and cost.

Surface preparation before coating: SSPC-SP 10 or ISO 8501-1 Sa 2.5 near-white blast, producing the cleanliness and anchor profile that determine coating service life.

Five control options: material selection, coatings and linings, cathodic protection, chemical inhibition, and environmental control such as dehumidification, oxygen removal, and pH adjustment.

Cathodic protection criteria: minus 0.85 V versus copper/copper sulfate for steel in soil or water, a 100 mV polarization shift, and minus 0.95 to minus 1.05 V in sulfate-reducing bacteria environments; overprotection risks hydrogen damage and coating disbondment.

Galvanic anodes of zinc, aluminium, or magnesium suit lower current demand and lower resistivity environments; impressed current systems use a rectifier and inert anodes where large, adjustable output is required.

Anodic inhibitors passivate the metal but cause severe localized attack if underdosed; cathodic inhibitors slow the reduction reaction; mixed inhibitors act on both half-reactions.

Risk-based inspection per API 580 and API 581 ranks equipment by likelihood and consequence of failure, then sets inspection method, coverage, and interval for the inspection plan.

Integrity operating windows define acceptable ranges for pH, temperature, velocity, and contaminant levels for each corrosion loop; excursions outside the window are documented and investigated.

Monitoring toolbox: weight-loss coupons give cumulative loss, electrical resistance probes work in conductive and non-conductive fluids, linear polarization resistance gives near-instantaneous rates in continuous water phases, ultrasonic thickness maps wall loss, hydrogen probes detect atomic hydrogen.

Coating failure modes include osmotic blistering, cathodic disbondment, chalking, cracking, and undercutting; holiday detection and dry film thickness measurement verify application quality.

Remaining life equals current thickness minus minimum required thickness, divided by corrosion rate; use the most severe credible rate and confirm the thinning mechanism before extending inspection intervals.

Buried pipeline assessment combines pipe-to-soil potentials, close interval surveys, direct current voltage gradient coating surveys, Wenner four-pin soil resistivity, and excavation at identified coating defects.

Non-destructive test selection: ultrasonic testing for wall thickness, radiography for internal discontinuities, magnetic particle for surface-breaking defects in ferromagnetic steel, dye penetrant for nonmagnetic surfaces, guided wave for long pipe runs.

Failure analysis sequence: gather operating history, document and sample the failure in situ, examine macro and microscopically, use SEM/EDS and XRD on corrosion products, perform metallography, then confirm the mechanism.

Microbiologically influenced corrosion evidence: black iron sulfide deposits, pitting beneath deposits, sulfide odour, sulfate-reducing bacteria enumerations, and localized attack in low-flow or stagnant sections.

Standards framework: API 510 for pressure vessels, API 570 for piping, API 653 for tanks, API 580 and 581 for risk-based inspection, NACE SP0169 for buried piping cathodic protection, NACE SP0188 for holiday detection.

Program leadership requires agreement among operations, inspection, materials, health and safety, and maintenance on damage mechanisms, inspection plans, and management-of-change reviews before any alteration is implemented.