AMPP O-CAT cheat sheet

Four conditions for corrosion: an anode, a cathode, an electrolyte path, and a metallic return path; remove any one and the cell stops.

Anodic reaction (oxidation) releases electrons: Fe -> Fe2+ + 2e-. Cathodic reaction (reduction) consumes them: O2 + 2H2O + 4e- -> 4OH- in neutral aerated water.

Galvanic series ranks alloys by practical potential in seawater; the farther apart two materials sit, the greater the driving voltage and the higher the anodic member's corrosion rate.

Noble-to-active seawater potentials: graphite and titanium near the noble end; carbon steel, cast iron, aluminum, zinc, and magnesium toward the active end.

Faraday's law: mass loss = (I x t x M)/(n x F), where F = 96,485 C/mol; a current of 1 A dissolving iron liberates about 9.1 kg per year at 100% efficiency.

Corrosion rate conversion: 1 mm/y equals 39.37 mpy; 1 microamp/cm2 of uniform anodic current on steel corresponds to roughly 0.0116 mm/y.

Pitting resistance equivalent number, PREN = %Cr + 3.3 x %Mo + 16 x %N; higher PREN indicates greater resistance to chloride pitting in stainless steels and duplex alloys.

Seawater velocity effects: velocities below about 1 m/s promote biofouling and under-deposit attack; sustained velocities above roughly 1.5-3 m/s (material dependent) promote erosion-corrosion and impingement.

Typical fixed jacket steels are normalized or TMCP grades such as API 2H, 2Y, and 2W; higher-strength grades carry reduced carbon equivalent to preserve weldability and toughness.

Carbon equivalent, CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15; values above about 0.40-0.43 increase hydrogen-assisted cold cracking risk in field welds.

Splash zone is the most aggressive external region: continuous wetting and drying, oxygen replenishment, tidal action, and impact; it is normally protected by a thick coating or sheathing rather than CP alone.

Submerged zone relies primarily on cathodic protection; the mudline and buried zone require allowance for anaerobic conditions and sulfate-reducing bacteria.

Atmospheric zone above the splash zone corrodes by airborne chloride deposition and humidity; coating selection follows ISO 12944 or NORSOK M-501 exposure categories.

Duplex stainless steels (about 22% Cr or 25% Cr) offer yield strengths near 450-550 MPa with chloride stress-corrosion resistance superior to 300-series austenitics.

Hydrogen-induced stress cracking risk rises with steel strength and cathodic overprotection; high-strength mooring chain and bolts require potential limits, often not more negative than about -0.95 V vs Ag/AgCl/seawater.

Fatigue life of tubular joints is governed by hot-spot stress and the S-N curve; weld toe grinding and post-weld heat treatment improve the classified fatigue category.

ISO 12944 corrosivity categories run C1 (very low, heated interiors) to C5 and CX (very high, offshore and tropical marine); offshore topsides typically specify C5 or CX.

NORSOK M-501 defines offshore coating systems by system number, for example System 1 for atmospheric epoxy and System 7 for thermally sprayed aluminum on carbon steel.

Surface preparation grade per ISO 8501-1: Sa 2 1/2 is near-white blast cleaning, the usual requirement for immersion and high-durability offshore systems.

Abrasive blast profile for high-build epoxy is typically 50-85 micrometers; profile must be measured and recorded, not assumed.

Thermally sprayed aluminum coatings on the splash zone act as a barrier and as a sacrificial anode; they remain effective when damaged because the aluminum galvanically protects exposed steel.

Coating holiday and defect density drives CP current demand; bare or damaged coating raises the current required and can cause overprotection of adjacent coated areas.

Epoxy-based systems are commonly used in immersion and splash zones; polyurethane and polysiloxane topcoats provide color and gloss retention in atmospheric zones.

Coating thickness is measured with magnetic or eddy-current gauges; a common acceptance rule is the 80-20 rule, where no reading falls below 80% of the specified nominal and the average meets nominal.

Protection criterion for carbon steel in seawater is typically -0.80 V vs Ag/AgCl/seawater, with -0.90 V often required for high-strength or stressed members.

Overprotection threshold on high-strength steel is commonly set at about -1.05 V vs Ag/AgCl/seawater to limit hydrogen evolution and cracking risk.

Sacrificial anodes are sized by total current demand, design life, and anode mass: mass = (mean current x life x 8760)/(anode capacity x utilization factor).

Typical anode capacities and closed-circuit potentials: aluminum-zinc-indium about 2,000 A-h/kg at -1.05 V; zinc about 780 A-h/kg at -1.03 V vs Ag/AgCl/seawater.

Current demand density varies by zone and coating condition: bare steel in the submerged zone is often taken as 0.10-0.15 A/m2, rising in cold or high-velocity water.

Impressed current systems use inert anodes and a transformer-rectifier; they suit large structures and high current demand but require power and careful potential control.

Anode current output is governed by Ohm's law: I = (Eanode - Eprotected)/(Ranode + Relectrolyte); anode resistance depends on anode geometry and seawater resistivity.

Retrofit CP design must account for existing coating degradation, remaining anode life, and any interaction with nearby structures or pipelines.

Risk-based inspection ranks equipment by likelihood and consequence of failure; inspection effort is directed to the highest-risk items.

Fitness-for-service assessment of corroded pipe commonly follows API 579-1/ASME FFS-1, using remaining wall thickness and remaining strength factor.

Remaining life estimate: remaining life = (current thickness - minimum required thickness)/corrosion rate; use the highest credible rate from recent inspection intervals.

Non-destructive testing methods: ultrasonic thickness gauging for wall loss, magnetic particle and dye penetrant for surface defects, radiography and phased-array UT for weld volumetric inspection.

Cathodic protection surveys use structure-to-electrolyte potential measurements, close-interval surveys, and current density checks; potentials more positive than criterion indicate inadequate protection.

Coating condition surveys record rust grade, blistering, cracking, and adhesion; ISO 4628 provides standard descriptors for defect type and severity.

Integrity management requires documented inspection history, anomaly tracking, and reassessment intervals tied to measured corrosion rates and consequence class.

Anode depletion is assessed by visual inspection, weight-loss sampling, or potential mapping; remaining anode life should exceed the planned next inspection interval.

Four conditions for corrosion: an anode, a cathode, an electrolyte path, and a metallic return path; remove any one and the cell stops.

Anodic reaction (oxidation) releases electrons: Fe -> Fe2+ + 2e-. Cathodic reaction (reduction) consumes them: O2 + 2H2O + 4e- -> 4OH- in neutral aerated water.

Galvanic series ranks alloys by practical potential in seawater; the farther apart two materials sit, the greater the driving voltage and the higher the anodic member's corrosion rate.

Noble-to-active seawater potentials: graphite and titanium near the noble end; carbon steel, cast iron, aluminum, zinc, and magnesium toward the active end.

Faraday's law: mass loss = (I x t x M)/(n x F), where F = 96,485 C/mol; a current of 1 A dissolving iron liberates about 9.1 kg per year at 100% efficiency.

Corrosion rate conversion: 1 mm/y equals 39.37 mpy; 1 microamp/cm2 of uniform anodic current on steel corresponds to roughly 0.0116 mm/y.

Pitting resistance equivalent number, PREN = %Cr + 3.3 x %Mo + 16 x %N; higher PREN indicates greater resistance to chloride pitting in stainless steels and duplex alloys.

Seawater velocity effects: velocities below about 1 m/s promote biofouling and under-deposit attack; sustained velocities above roughly 1.5-3 m/s (material dependent) promote erosion-corrosion and impingement.

Typical fixed jacket steels are normalized or TMCP grades such as API 2H, 2Y, and 2W; higher-strength grades carry reduced carbon equivalent to preserve weldability and toughness.

Carbon equivalent, CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15; values above about 0.40-0.43 increase hydrogen-assisted cold cracking risk in field welds.

Splash zone is the most aggressive external region: continuous wetting and drying, oxygen replenishment, tidal action, and impact; it is normally protected by a thick coating or sheathing rather than CP alone.

Submerged zone relies primarily on cathodic protection; the mudline and buried zone require allowance for anaerobic conditions and sulfate-reducing bacteria.

Atmospheric zone above the splash zone corrodes by airborne chloride deposition and humidity; coating selection follows ISO 12944 or NORSOK M-501 exposure categories.

Duplex stainless steels (about 22% Cr or 25% Cr) offer yield strengths near 450-550 MPa with chloride stress-corrosion resistance superior to 300-series austenitics.

Hydrogen-induced stress cracking risk rises with steel strength and cathodic overprotection; high-strength mooring chain and bolts require potential limits, often not more negative than about -0.95 V vs Ag/AgCl/seawater.

Fatigue life of tubular joints is governed by hot-spot stress and the S-N curve; weld toe grinding and post-weld heat treatment improve the classified fatigue category.

ISO 12944 corrosivity categories run C1 (very low, heated interiors) to C5 and CX (very high, offshore and tropical marine); offshore topsides typically specify C5 or CX.

NORSOK M-501 defines offshore coating systems by system number, for example System 1 for atmospheric epoxy and System 7 for thermally sprayed aluminum on carbon steel.

Surface preparation grade per ISO 8501-1: Sa 2 1/2 is near-white blast cleaning, the usual requirement for immersion and high-durability offshore systems.

Abrasive blast profile for high-build epoxy is typically 50-85 micrometers; profile must be measured and recorded, not assumed.

Thermally sprayed aluminum coatings on the splash zone act as a barrier and as a sacrificial anode; they remain effective when damaged because the aluminum galvanically protects exposed steel.

Coating holiday and defect density drives CP current demand; bare or damaged coating raises the current required and can cause overprotection of adjacent coated areas.

Epoxy-based systems are commonly used in immersion and splash zones; polyurethane and polysiloxane topcoats provide color and gloss retention in atmospheric zones.

Coating thickness is measured with magnetic or eddy-current gauges; a common acceptance rule is the 80-20 rule, where no reading falls below 80% of the specified nominal and the average meets nominal.

Protection criterion for carbon steel in seawater is typically -0.80 V vs Ag/AgCl/seawater, with -0.90 V often required for high-strength or stressed members.

Overprotection threshold on high-strength steel is commonly set at about -1.05 V vs Ag/AgCl/seawater to limit hydrogen evolution and cracking risk.

Sacrificial anodes are sized by total current demand, design life, and anode mass: mass = (mean current x life x 8760)/(anode capacity x utilization factor).

Typical anode capacities and closed-circuit potentials: aluminum-zinc-indium about 2,000 A-h/kg at -1.05 V; zinc about 780 A-h/kg at -1.03 V vs Ag/AgCl/seawater.

Current demand density varies by zone and coating condition: bare steel in the submerged zone is often taken as 0.10-0.15 A/m2, rising in cold or high-velocity water.

Impressed current systems use inert anodes and a transformer-rectifier; they suit large structures and high current demand but require power and careful potential control.

Anode current output is governed by Ohm's law: I = (Eanode - Eprotected)/(Ranode + Relectrolyte); anode resistance depends on anode geometry and seawater resistivity.

Retrofit CP design must account for existing coating degradation, remaining anode life, and any interaction with nearby structures or pipelines.

Risk-based inspection ranks equipment by likelihood and consequence of failure; inspection effort is directed to the highest-risk items.

Fitness-for-service assessment of corroded pipe commonly follows API 579-1/ASME FFS-1, using remaining wall thickness and remaining strength factor.

Remaining life estimate: remaining life = (current thickness - minimum required thickness)/corrosion rate; use the highest credible rate from recent inspection intervals.

Non-destructive testing methods: ultrasonic thickness gauging for wall loss, magnetic particle and dye penetrant for surface defects, radiography and phased-array UT for weld volumetric inspection.

Cathodic protection surveys use structure-to-electrolyte potential measurements, close-interval surveys, and current density checks; potentials more positive than criterion indicate inadequate protection.

Coating condition surveys record rust grade, blistering, cracking, and adhesion; ISO 4628 provides standard descriptors for defect type and severity.

Integrity management requires documented inspection history, anomaly tracking, and reassessment intervals tied to measured corrosion rates and consequence class.

Anode depletion is assessed by visual inspection, weight-loss sampling, or potential mapping; remaining anode life should exceed the planned next inspection interval.