Corrosion & Cathodic Protection

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Ons and offs at MP 14 are -1.1 and -0.82, we're low, check the casing at the county road for a short.

  TranslationPaired on and instant-off readings show the pipe falls just short of the -850 mV criterion, so test whether the road crossing casing is touching the carrier pipe.

• Jeep found six holidays on the field joints, patch them and re-jeep before the sideboom picks it up.

  TranslationHigh-voltage holiday testing found six flaws in the girth weld coatings; repair them and retest before the pipe is lifted into the trench.

• Steel's inside the dew point spread and yesterday's blast has flash rust, Bresle it before anybody sprays.

  TranslationThe steel is too close to the dew point and the previously blasted surface has rusted lightly, so test for soluble salt contamination before coating.

• ILI call and the DCVG both point at the same spot, open a bell hole and send any cut-out to the lab.

  TranslationThe in-line inspection tool and the above-ground coating survey agree on one location, so excavate it, and if pipe is removed, send it for analysis.

• Bug bottles went four deep and the pig brought back black powder, bump the biocide and pull the coupons.

  TranslationBacteria cultures show high counts and pigging recovered iron sulfide dust, so increase the biocide treatment and check the corrosion coupons.

• Rec's on the top tap and still can't hold -850 off, either the groundbed's gas-blocked or the coating's shot.

  TranslationThe rectifier is at maximum output and still cannot reach the protection criterion, pointing to a high-resistance anode bed or widespread coating breakdown.

• Give the epoxy its sweat-in, stripe the welds and check DFT before the urethane goes on.

  TranslationLet the mixed epoxy stand for its induction time, brush an extra coat onto the welds, and measure dry film thickness before the topcoat.

• Instant-off is -1350 by the groundbed, that's overprotection, tap it down before we get cathodic disbondment.

  TranslationThe polarized potential near the anode bed is far too negative, so reduce rectifier output before the coating lifts off around its holidays.

The forms corrosion takes, from even rusting to pits, cracks and microbial attack, and the field names for them.

• Black powderalso iron sulfide dust
  Fine black dust of iron sulfides and oxides that forms inside gas pipelines and fouls filters, meters and valves. Shop talk
• Bugsalso bacteria, microbes
  Field slang for the bacteria behind microbiologically influenced corrosion and system souring. Shop talk
• Corrosionalso metal loss
  Metal wasting away as it reacts with the air, water or soil around it.
• Corrosion fatiguealso CF
  Cracking under cyclic stress in a corrosive environment, failing at lower stress or fewer cycles than in air.
• Crevice corrosionalso concentration cell corrosion, gasket face corrosion
  Localized attack inside tight gaps under gaskets, deposits or lap joints where the solution stagnates.
• CUIalso corrosion under insulation
  Corrosion under insulation: hidden external corrosion of piping and vessels beneath wet thermal insulation.
• Dealloyingalso selective leaching, dezincification
  Selective leaching of one element from an alloy, leaving a weak, porous remnant that still looks intact.
• Erosion-corrosionalso impingement attack, erosion corrosion
  Accelerated metal loss where flowing fluid strips protective films and corrosion follows, often at bends and tees.
• Galvanic corrosionalso bimetallic corrosion, dissimilar metal corrosion
  Accelerated corrosion of the more active metal when two dissimilar metals touch in a shared electrolyte.
• Galvanic seriesalso practical galvanic series
  Ranking of real metals and alloys by their measured potential in a given electrolyte, usually seawater.
• Hydrogen embrittlementalso HE, hydrogen-assisted cracking
  Loss of ductility when atomic hydrogen gets into steel, leading to sudden cracking under stress.
• Intergranular corrosionalso IGC, intergranular attack
  Attack that runs along grain boundaries, often in stainless steel sensitized by welding heat.
• MICalso microbial corrosion, biocorrosion
  Microbiologically influenced corrosion: corrosion started or sped up by bacteria and other microbes.
• Pittingalso pitting corrosion, pits
  Localized corrosion that drills small, deep cavities into metal while the surface around them stays nearly intact.
• PRENalso PRE, pitting resistance equivalent
  Pitting resistance equivalent number: an index of a stainless alloy's chloride pitting resistance from its chemistry.
• Red wateralso rusty water, discolored water
  Rust-colored tap or process water carrying iron corrosion products shed from pipes and mains. Shop talk
• Rust jackingalso pack rust, rust packing
  Swelling of rust between steel parts that pries joints apart, bows plates and pops rivets. Shop talk
• SCCalso stress corrosion cracking
  Stress corrosion cracking: brittle cracking caused by tensile stress acting together with a specific corrosive environment.
• Sour servicealso sour, H2S service
  Operating conditions with wet hydrogen sulfide, which call for cracking-resistant materials and controls.
• Sulfide stress crackingalso SSC, sulfide cracking
  Cracking of hard steel under tension in wet hydrogen sulfide service, a form of hydrogen embrittlement.
• Tea stainingalso rust bloom on stainless
  Light brown surface discoloration on stainless steel exposed to salt air or pollution, mostly cosmetic. Shop talk
• Tuberculationalso tubercles, tuberculated pipe
  Growth of knob-like mounds of rust and deposits on the inside of iron and steel water pipes.
• Uniform corrosionalso general corrosion, general attack
  Corrosion spread evenly over a whole surface, thinning the metal at a fairly predictable rate.

The cells, potentials and reference electrodes behind every CP reading, and the numbers that define protection.

• 100 mV polarization criterionalso 100 mV shift, 100 mV decay
  CP criterion met when a structure is polarized at least 100 mV more negative than its native potential.
• -850 mV criterionalso -0.85 V criterion, eight-fifty
  Steel counts as protected when its potential is -850 mV or more negative against a copper sulfate electrode.
• Anodealso anodic site
  Electrode where oxidation happens and metal dissolves; in CP, the hardware installed to discharge current.
• Cathodealso cathodic site
  Electrode where reduction happens and current is collected, so the metal there does not corrode.
• CSEalso Cu/CuSO4, copper sulfate electrode
  Copper-copper sulfate electrode, the standard reference for pipe-to-soil potential readings.
• Electrolyte
  Conductive medium such as soil, seawater or concrete pore water that carries ionic current between anode and cathode.
• Half-cellalso cell, copper cell
  Field name for the portable reference electrode that CP techs carry to take potential readings. Shop talk
• Instant-off potentialalso instant off, off potential
  Structure potential read in the split second after CP current is switched off, free of IR drop.
• IR dropalso ohmic drop, IR error
  Voltage error in a potential reading caused by current flowing through the soil between pipe and electrode.
• Lowalso under criterion, under-protected
  CP tech shorthand for a potential that is not negative enough to meet the protection criterion. Shop talk
• mpyalso mils per year
  Mils per year, the US corrosion rate unit: thousandths of an inch of metal lost per year.
• Native potentialalso corrosion potential, free-corrosion potential
  Potential of a buried or immersed structure before any cathodic protection current is applied.
• Overprotectionalso over-protection, excessive polarization
  So much cathodic current that potentials get too negative, risking coating damage and hydrogen problems.
• Polarizationalso cathodic polarization, polarization shift
  Shift of a metal's potential away from its native value caused by current flowing across its surface.
• Reference electrodealso reference cell, ref cell
  Electrode with a stable, known potential that serves as the fixed zero for measuring structure potentials.
• Soil resistivityalso resistivity, Wenner four-pin
  How strongly soil resists electric current, in ohm-cm, the main guide to soil corrosivity and anode design.

Anodes, rectifiers, groundbeds, bonds and the rest of the hardware that pushes protective current onto steel.

• Bondalso resistance bond, continuity bond
  Wire or cable connection that ties two metal structures together electrically, often to control CP current.
• Cad weldalso cadweld, thermite weld
  Shop name for an exothermic weld, the thermite connection used to attach CP cables to pipe and anodes. Shop talk
• Cathodic protectionalso CP
  Shielding buried or submerged steel from corrosion with DC current from sacrificial anodes or a rectifier.
• Coke breezealso coke backfill, carbonaceous backfill
  Crushed carbon backfill packed around impressed-current anodes to lower resistance and extend anode life.
• Decoupleralso solid-state decoupler, SSD
  Solid-state device that blocks DC but passes AC and fault current, grounding a pipeline without losing CP.
• Deep anodealso deep well groundbed, deep anode bed
  Impressed-current groundbed with anodes stacked in a vertical hole drilled tens of metres deep or more.
• Groundbedalso anode bed, ground bed
  Group of impressed-current anodes buried in backfill that discharges rectifier current into the earth.
• ICCPalso impressed current, IC system
  Impressed current cathodic protection: CP powered by a rectifier pushing current through long-life anodes.
• Insulating jointalso isolation joint, insulating flange kit
  Fitting that electrically isolates one pipe section from another so CP current stays where it belongs.
• Interrupteralso current interrupter, GPS interrupter
  Timed switch fitted to a rectifier or bond that cycles CP current on and off for instant-off readings.
• Mag anodealso mags, magnesium anode
  Field name for a magnesium sacrificial anode, the high-voltage choice for high-resistivity soils. Shop talk
• MMOalso mixed metal oxide, MMO anode
  Mixed metal oxide anode: titanium coated with precious-metal oxides, the standard long-life ICCP anode.
• Rectifieralso rec, rect
  Transformer-rectifier unit that turns AC power into adjustable DC current for an impressed current CP system.
• Sacrificial anodealso galvanic anode, packaged anode
  Block of a more active metal such as magnesium, zinc or aluminum that corrodes to protect connected steel.
• Shorted casingalso casing short, metallic short
  Road or rail crossing casing pipe that has made metal contact with the carrier pipe and steals CP current. Shop talk
• Shuntalso current shunt, meter shunt
  Precision low-resistance resistor in a CP circuit, used to measure current from the voltage across it.
• Tapalso tap setting, coarse tap
  Adjustable transformer setting on a rectifier, used to raise or lower DC output in coarse and fine steps.
• Test stationalso test post, test point
  Above-ground post or box with wires bonded to a buried structure, used to take CP readings.
• Zincsalso zinc anodes, zinc
  Boat and marine slang for the sacrificial anodes bolted to hulls, shafts, outdrives and heat exchangers. Shop talk

How crews measure protection along a line, find coating damage and dig up what the tools flag.

• Bell holealso integrity dig, dig
  Excavation dug around a short section of buried pipe so crews can inspect, repair or connect to it. Shop talk
• CISalso close interval survey, CIPS
  Close interval survey: potential readings taken every metre or so along a pipeline to find protection gaps.
• CP couponalso cathodic protection coupon, probe coupon
  Small bare steel sample buried beside a pipe and wired to it, used to read IR-free potentials.
• CP techalso CP technician, corrosion tech
  Cathodic protection technician who reads rectifiers, runs surveys and troubleshoots protection on pipes and tanks. Shop talk
• Cut-outalso cutout, pipe sample
  Section of pipe removed from service so corrosion damage can be examined, tested or replaced. Shop talk
• DCVGalso DC voltage gradient survey
  Direct current voltage gradient survey that locates and sizes coating holidays on buried pipe.
• Foreign linealso foreign structure, foreign pipeline
  Field term for another operator's pipeline or buried metal structure near or crossing your own. Shop talk
• Hot spotalso hot-spot protection, active corrosion area
  Localized area of active corrosion on a pipeline, or the targeted anodes installed to protect just that spot. Shop talk
• ILIalso in-line inspection, intelligent pigging
  In-line inspection: sending instrumented tools through a pipeline to measure wall loss, dents and cracks.
• Interferencealso stray current, stray current corrosion
  Unwanted DC or AC current picked up from another source, which corrodes a structure where it leaves.
• Ons and offsalso on/offs, on and off readings
  Survey slang for paired on and instant-off potential readings taken while rectifiers are being interrupted. Shop talk
• Pipe-to-soil potentialalso P/S, PSP
  Voltage between a buried pipe and a reference electrode on the soil surface, the basic CP measurement.
• Smart pigalso intelligent pig, ILI tool
  Industry nickname for an in-line inspection tool that rides through a pipeline recording its condition. Shop talk

Blast standards, coating systems, inspection checks and the defects coating inspectors chase.

• Amine blushalso blush, blushing
  Waxy, greasy film on curing amine epoxies in cold or damp conditions that blocks adhesion of the next coat.
• Anchor profilealso surface profile, profile
  Peak-to-valley roughness that blasting cuts into steel so a coating can grip, given in mils or microns.
• Black beautyalso coal slag, slag abrasive
  Genericized name for black coal slag abrasive used in open blasting of steel. Shop talk
• Blast potalso pot, blast machine
  Pressurized tank that meters abrasive into the air stream for blasting, controlled from the nozzle. Shop talk
• Cathodic disbondmentalso CD, cathodic disbonding
  Loss of coating adhesion around a holiday caused by the alkali and hydrogen that cathodic protection generates.
• Chalkingalso chalk
  Powdery residue on an aging coating surface as sunlight breaks down the binder.
• Dew pointalso dew point spread, 5 degree rule
  Temperature at which moisture condenses from air; specs keep steel at least 3 C above it for blasting and painting.
• DFTalso dry film thickness, dry mils
  Dry film thickness: the thickness of a cured coating layer, measured in mils or microns.
• Dopealso coal tar enamel, hot dope
  Pipeline slang for hot-applied coal tar or asphalt enamel coating, poured over the pipe and wrapped. Shop talk
• FBEalso fusion-bonded epoxy, fusion bond
  Fusion-bonded epoxy: a powder coating fused onto heated pipe, the standard external line pipe coating.
• Field joint coatingalso girth weld coating, field joint
  Coating applied on site over the bare girth weld zone left between factory-coated pipe joints.
• Fish eyealso fisheye, cratering
  Small round crater in wet paint where the coating pulls away from a spot of oil or silicone contamination. Shop talk
• Flash rustalso flash rusting, rust bloom
  Thin, quick layer of rust that forms on freshly blasted or water-washed steel within minutes to hours.
• Holidayalso coating holiday, pinhole
  Pinhole, void or bare spot in a coating that leaves the metal underneath exposed.
• Holiday detectoralso spark tester, wet sponge tester
  Instrument that finds coating holidays by detecting current flow or sparks through flaws to the metal.
• Hot-dip galvanizingalso galv, HDG
  Coating steel by dipping it in molten zinc, forming bonded zinc-iron layers that protect by barrier and sacrifice.
• Jeepalso jeeping, jeep spring
  Pipeline slang for the high-voltage holiday detector, and also for the coating flaws it finds. Shop talk
• Mill scalealso millscale, scale
  Blue-black iron oxide skin formed on steel during hot rolling, which must come off before painting.
• Mud crackingalso mudcracking
  Network of cracks like dried mud in a coating applied too thick, typical of inorganic zinc and waterborne paints.
• Near-white blastalso SP 10, NACE No. 2
  Blast grade allowing light shadows or stains on no more than 5 percent of the surface, the usual immersion standard.
• Pot lifealso working life, usable life
  Working time after mixing a two-part coating before it has reacted too far to apply properly.
• Pull-off adhesion testalso pull-off test, dolly test
  Test that glues a dolly to a coating and pulls it off to measure how strongly the coating sticks.
• Sandblastingalso blasting, abrasive blasting
  Everyday word for abrasive blast cleaning, used even when no sand is involved. Shop talk
• Soluble saltsalso chlorides, salt contamination
  Invisible chloride, sulfate and nitrate contamination on steel that causes blistering and early coating failure.
• Stripe coatalso striping, stripe
  Extra coat brushed onto edges, welds, bolts and corners to build thickness where spray runs thin.
• Sweat-inalso induction time, induction period
  Painter slang for the induction time a freshly mixed epoxy must stand before it is applied. Shop talk
• Toothalso bite, key
  Painters' word for the roughness on a surface that gives a coating something to grip. Shop talk
• White metal blastalso SP 5, NACE No. 1
  Top abrasive blast cleaning grade: bare, uniform steel with no visible residue, rust or staining.
• Zinc-rich primeralso zinc primer, IOZ
  Primer loaded with zinc dust that protects steel galvanically as well as acting as a barrier.

Coupons, probes and chemicals used to measure and slow corrosion inside pipes, vessels and water systems.

• Bug bottlealso serial dilution vials, culture bottles
  Field nickname for the serial dilution culture vials used to count bacteria in water samples. Shop talk
• Corrosion allowancealso CA, corrosion margin
  Extra wall thickness added beyond what pressure design needs, meant to be used up by corrosion over the service life.
• Corrosion couponalso weight-loss coupon, mass-loss coupon
  Weighed metal strip exposed inside a system for a set time, then reweighed to calculate corrosion rate.
• Corrosion inhibitoralso inhibitor, CI
  Chemical added in small doses to a fluid to slow corrosion of the metal it touches.
• ER probealso electrical resistance probe, ER
  Electrical resistance probe that tracks corrosion by measuring the rising resistance of a thinning metal element.
• LPRalso linear polarization resistance, LPR probe
  Linear polarization resistance: an electrochemical method that reads corrosion rate in minutes in conductive liquids.
• Oxygen scavengeralso scavenger, O2 scavenger
  Chemical that strips dissolved oxygen from water to stop oxygen pitting in boilers and injection systems.
• SRBalso sulfate-reducing bacteria, sulfate reducers
  Sulfate-reducing bacteria: oxygen-free microbes that make hydrogen sulfide and drive much MIC.

Frequently asked questions

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