AMPP Corrosion Specialist flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Corrosion cell components

Four elements must coexist for electrochemical corrosion: an anode where metal oxidizes, a cathode where reduction occurs, an electrolyte conducting ions between them, and a metallic path carrying electrons. Eliminating any one arrests the corrosion.

Faraday's law

The mass of metal dissolved is proportional to current multiplied by time: m equals M times I times t divided by n times F, where F is 96,485 coulombs per mole.

Nernst equation

At 25 degrees C, E equals E-standard minus 0.0592 divided by n, multiplied by the logarithm of the reaction quotient. It gives the actual half-cell potential at the prevailing ion activity.

Pourbaix diagram

A potential versus pH map showing thermodynamic regions of immunity, passivity, and corrosion for a metal in water. It predicts tendency to corrode but provides no kinetic rate information.

Differential aeration cell

Corrosion driven by oxygen concentration differences. The oxygen-depleted region acts as the anode and corrodes, while the oxygen-rich region acts as the cathode. It commonly occurs under deposits and in crevices.

Pitting potential

The potential above which a passive metal undergoes localized breakdown and pitting in a given environment. Higher values indicate greater resistance to chloride attack and a wider passive range.

Stress corrosion cracking

Cracking produced by the simultaneous action of tensile stress and a specific corrosive environment, such as chloride for austenitic stainless steel, caustic for carbon steel, or ammonia for brass.

Dezincification

Selective leaching of zinc from brass, leaving a porous copper-rich residue with little strength. It occurs in copper-zinc alloys exposed to stagnant, low-flow water with high chloride or low pH.

Corrosion rate unit conversion

One mil per year equals 0.0254 millimetres per year. A uniform current density of one microamp per square centimetre corresponds to about 11.6 micrometres per year for iron.

Galvanic series

A ranking of metals and alloys by their open-circuit potentials measured in seawater. Coupling alloys far apart in the series increases galvanic current and accelerates attack on the more active material.

Pitting resistance equivalent number

Calculated as percent chromium plus 3.3 times percent molybdenum plus 16 times percent nitrogen. Values above roughly 40 indicate good resistance to chloride pitting in seawater and marine service.

Anodic area ratio effect

When a small anode is coupled to a large cathode, the cathodic current concentrates on a small area, producing rapid deep penetration. Design should maximize anodic area and minimize cathodic area.

Corrosion allowance

Extra wall thickness added to a component so that predictable uniform metal loss over the design life does not breach the minimum required thickness. It does not protect against pitting or cracking.

Sour service limits

NACE MR0175 and ISO 15156 apply when hydrogen sulfide partial pressure exceeds 0.00035 MPa (0.05 psi). Carbon steels are then restricted to a maximum hardness of 22 HRC.

Duplex stainless properties

Grades with a mixed ferritic-austenitic microstructure giving roughly twice the yield strength of austenitic grades, with high chromium and molybdenum for chloride resistance in marine and process service.

Dissimilar metal insulation

Where different metals must be joined, an insulating gasket, washer, or transition piece breaks the electrical path, preventing galvanic current from driving corrosion of the more active alloy.

Cathodic protection current demand

Bare steel in flowing seawater needs roughly 100 to 110 milliamps per square metre, whereas a well-coated structure may need only 1 to 25 milliamps per square metre. Coating condition drives anode sizing.

Cathodic protection criterion

A pipe-to-electrolyte potential of minus 0.85 volts or more negative, measured against a copper/copper sulfate reference electrode, is the accepted protection criterion for steel in soil and natural waters.

100 mV polarization criterion

A potential shift of at least 100 millivolts from the free-corrosion potential, in either the cathodic or anodic direction, indicates adequate cathodic protection of buried or submerged steel.

Impressed current cathodic protection

A rectifier supplies direct current to inert anodes, delivering high, adjustable protection current over large structures. It can cause stray-current interference and overprotection if not controlled.

Galvanic anode cathodic protection

Sacrificial zinc, aluminium, or magnesium anodes corrode to supply protective current to the structure. The system is simple and self-regulating but limited in current output and environment resistivity.

Anodic inhibitor

A chemical that forms or thickens a passive film over anodic areas. Underdosing is dangerous because the remaining unprotected anodes experience concentrated, rapid localized attack.

Risk based inspection

An API 580 and 581 methodology ranking equipment by the likelihood and consequence of failure, then assigning inspection methods, coverage, and intervals proportionate to the calculated risk.

Integrity operating window

A defined range for process parameters such as temperature, pH, velocity, and contaminant concentration within which identified corrosion mechanisms remain at an acceptable rate. Exceedances trigger documented action.

Linear polarization resistance

A technique applying a small potential perturbation and measuring the resulting current to obtain an instantaneous corrosion rate. It requires a continuous, conductive aqueous phase to work reliably.

Remaining life calculation

Current measured thickness minus minimum required thickness, divided by the corrosion rate, gives remaining service life. The rate used should reflect the most severe credible damage mechanism.

Half-cell potential survey

Measurement of structure-to-electrolyte potentials with a portable reference electrode, usually at regular intervals along a buried pipeline, to map cathodic protection effectiveness and locate coating defects.

Close interval survey

Pipe-to-soil potential readings taken at one to three metre intervals with cathodic protection current interrupted, resolving protection gaps and coating defects that widely spaced surveys would miss.

Wenner four-pin method

A soil resistivity test injecting current through outer pins and measuring voltage between inner pins. Resistivity indicates soil corrosivity and guides galvanic anode groundbed design and placement.

Direct current voltage gradient

A coating survey that maps voltage gradients in the soil above a buried pipeline while cathodic protection current flows, pinpointing coating holidays and areas of active corrosion.

Microbiologically influenced corrosion

Deterioration caused or accelerated by microorganisms, typically sulfate-reducing bacteria. It produces localized pitting, black iron sulfide deposits, sulfide odour, and attack in low-flow sections.

Failure analysis sequence

A structured investigation: collect background and operating history, document and preserve the failure, sample and photograph, perform laboratory examination, then confirm the mechanism before recommending corrective action.