Corrosion & Coatings Glossary

A condition monitoring location (CML) is a designated point on a piping system where wall thickness and condition are measured repeatedly over time, so corrosion rates and remaining life can be calculated. CML placement is risk-based — elbows, injection points, dead legs and interfaces get priority.

A dead leg is a section of piping with no or intermittent flow — bypasses, drains, spare pump suctions, capped future connections. Stagnation lets water, solids and bacteria accumulate, so dead legs routinely corrode at several times the rate of the flowing circuit they branch from.

The dew point is the temperature at which airborne moisture condenses on a surface. In coating work, the standing rule is that the substrate must stay a margin above the dew point — commonly 3 °C / 5 °F — during application and initial cure, or an invisible film of condensation compromises adhesion.

Dry film thickness (DFT) is the thickness of a coating after it has fully cured, measured from the substrate surface to the top of the film. It is the single most-checked quantity in coating quality control, because both under-thickness and over-thickness shorten coating life.

A holiday test is an electrical inspection that locates discontinuities — pinholes, voids, thin spots — in a nominally continuous coating. A detector applies a voltage across the film; where current passes to the substrate, the film is breached, and the spot is marked for repair.

Passivation is the state — and the chemical treatment that restores it — in which stainless steel is protected by its thin chromium-oxide surface film. The treatment dissolves free iron and contamination so the film can reform uniformly; a stainless surface that loses its film locally corrodes like ordinary steel there.

Pitting is a localized corrosion mechanism in which attack concentrates on small points of a metal surface and penetrates inward, producing deep, narrow cavities while overall metal loss stays negligible. It is autocatalytic — a growing pit creates the acidic, chloride-rich local chemistry that accelerates its own growth.

The pitting resistance equivalent number (PREN) ranks stainless steels and related alloys by their intrinsic resistance to pitting corrosion, computed from composition — most commonly PREN = %Cr + 3.3×%Mo + 16×%N. Higher is more resistant; typical values run from about 19 (304) to over 40 (super duplex).

A sacrificial anode is a piece of deliberately active metal — zinc, aluminum or magnesium — electrically connected to a structure so that galvanic action corrodes the anode instead of the structure. It is galvanic corrosion used on purpose, and it only works while anode material remains and the connection is sound.

Sulfate-reducing bacteria (SRB) are anaerobic microorganisms that respire sulfate instead of oxygen, producing hydrogen sulfide. Living in the oxygen-free zone under biofilms and deposits, they are the best-known drivers of microbiologically influenced corrosion (MIC), leaving black sulfide deposits and deep localized pits.

Surface profile — also called anchor pattern — is the peak-to-valley roughness left on a metal surface by abrasive blasting or power tooling. Coatings grip these peaks mechanically, so profile depth directly governs adhesion; a profile too shallow underperforms, one too deep leaves peaks under-covered.