Metal Glossary


Treatment in oxidising acids to remove light surface contamination by iron based materials left by machining or wire brushing and to enhance the naturally occurring chromium oxide film. Any residual contaminants can act as sites for corrosion cells. Heavy contamination or other contaminants (mill scale, etc.) should be removed by acid pickling before passivation.

Passive Film, Oxide Film

Generally it is the oxide layer spontaneously formed on the surface of a metal or alloy by its reaction with the oxygen in the atmosphere.
Specifically in stainless steels it is the protective chromium oxide film that forms on alloys containing more than 10.5% chromium that is impervious to many corroding media. It is why stainless steels are “stainless”.


Any constituent part of an alloy, e.g. elemental metal, solid solution, or inter-metallic compound, that is physically and chemically homogeneous and distinct within the microstructure.

Physical Properties

The intrinsic properties of a material, e.g. density, electrical conductivity, thermal conductivity, thermal expansion that govern its behaviour and potential for use in an application.

Pickling, Acid Pickling

The removal, by immersion in a solution containing nitric and hydrofluoric acids, of the scale left after hot working or annealing. It leaves a matt grey finish.

Related terms: Passivation

Pinch Passing, Skin Passing

A cold rolling pass, with minimal reduction of thickness, of annealed sheet or strip. It reduces the tendency to kinks, flats and stretcher strains on subsequent manipulation. Flat material so treated is described as 'skin passed', 'pinch passed', 'non-kinking', 'non-flatting' or 'killed'. The material will also be brighter and have closer tolerances.

Pipes, Pipe

Tubular products made to Nominal Bore and Schedule Wall Thickness dimensions specified by ANSI/ASTM standards. Sizes specifically for Stainless Steel Pipe are to ANSI B36.19 where the Schedule Wall Thickness has an S after the number to denote Stainless – e.g. Schedule 40S. Stainless Steel Pipe can also be made to the ANSAI B36.10 sizes intended for carbon/mild/alloy steels where the schedule has no S after it.


A corroded hollow in a metal surface, caused by localised corrosion (pitting).

Pitting Corrosion

Concentrated, localised, corrosion attack that can reach considerable depths and cause premature failure.

Precipitation Hardening Stainless Steels

Precipitation hardening stainless steels are Iron, Nickel, Chromium alloys that also contain precipitation hardening additions such as Aluminium, Titanium, Copper, Niobium and Molybdenum. They offer higher mechanical properties than the austenitic grades, with lower toughness and do have comparable corrosion resistance.

Pressure Test

A hydraulic or pneumatic test for tubes to prove that the material can withstand a specified pressure for a specified time without leakage or rupture.

Proof Strength

The tensile load required to extend a sample of material by 0.2% (RP0.2) or 1% (RP1.0). Measured in Newtons per square mm (Nmm-2) or Mega-Pascals (MPa).