Corrosion Glossary - H

Handhole: an access opening in a pressure part usually not exceeding 17 cm in its longest dimension.
Handhole cover: a handhole closure.
Hardener: curing agent for epoxies or fiberglass.
Hard chromium plating: the electrolytic deposition of chromium to form a very hard (1000 Hv), tough coating with good wear resistance. The structure is micro-cracked.
Hard water: water which contains calcium or magnesium in an amount which require an excessive amount of soap to form a lather.
Hardness: a measure of the amount of calcium and magnesium salts in water. usually expressed as grains per gallon or ppm as calcium carbonate (CaCO₃).
Hardness test: a test designed to assess the resistance to penetration from a load. The surface is indented under a defined load and the depth or area of penetration is measured.
Hardfacing: the application of a cladding or coating of material designed to resist wear.
Haze: a general term applied to virtually any dulling of a paint surface. It is used to cover a wide variety of defects. Haze may be due to material that has exuded to the surface, fine solvent popping, roughening of the surface because of flocculation or poor coalescence, microvoids, etc.
Hazop: a structured process intended to proactively identify equipment modifications and/or safety devices required in order to avoid any significant safety or environmental incident as a result of equipment failure.
Head: a measure of the pressure at a point in a water system.
Head loss: the same as "pressure drop".
Heat balance: an accounting of the distribution of the heat input, output and losses.
Heat exchanger: a vessel in which heat is transferred from one medium to another.
Heat release rate: rate that describes the heat available per square foot of heat-absorbing surface in the furnace or per cubic foot of volume.
Heating surface: those surfaces which are exposed to products of combustion on one side and water on the other. This surface is measured on the side receiving the heat.
Heating value: the quantity of heat released by a fuel through complete combustion. it is commonly expressed in btu per lb, per gallon, or cu-ft.
Helium (He): monatomic noble gas, Most inert element, atomic number 2. Used as a plasma gas in plasma spraying.
Hexavalent chromium: or chromium (VI)
Hiding power: the ability of a paint to mask the color or pattern of a surface.
High velocity oxy-fuel spraying (HVOF): a Thermal spray process. The spray powder particles are injected into a high velocity jet formed by the combustion of oxygen and fuel, heated and accelerated to the workpiece.
Hot corrosion: an accelerated corrosion of metal surfaces that results from the combined effect of oxidation and reactions with sulfur compounds and other contaminants, such as chlorides, to form a molten salt on a metal surface which fluxes, destroys, or disrupts the normal protective oxide.
Hot cracking: also called solidification cracking. Hot cracking of weldments is caused by the segregation at grain boundaries of low-melting constituents in the weld metal. Hot cracking can be minimized by the use of low-impurity welding materials and proper joint design.
Hot dip coating: a metallic coating obtained by dipping the base metal or substrate into a molten metal.
Hot isostatic processing (HIPping): the high temperature/high pressure consolidation of a powder metallurgy component or thermally sprayed coating. Density is greatly increased and metallurgical changes provide enhanced corrosion and wear properties.
Hot-rolling, hot-rolled steel (HR): rolling steel slabs into flat-rolled steel after it has been reheated.
Hot shortness: a tendency for some alloys to separate along grain boundaries when stressed or deformed at temperatures near the melting point. Hot shortness is caused by a low-melting constituent, often present only in minute amounts, that is segregated at grain boundaries.
Hot working: deforming metal plastically at such a temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening.
Huey test: corrosion testing in a boiling solution of nitric acid. This test is mainly used to detect the susceptibility to intergranular corrosion of stainless steel.
Humidity test: a corrosion test involving exposure of specimens at controlled levels of humidity and temperature.
HVOF: See high velocity oxygen fuel spraying.
Hydration: the chemical combination of water into a substance.
Hydrocarbon: a chemical compound of hydrogen and carbon.
Hydrogen: diatomic gas, atomic number 1, The lightest element, very reactive and powerful reducing agent. Used as a secondary plasma gas in the plasma spraying process and as a fuel gas in combustion thermal spray processes (CWS, CPS and HVOF).
Hydrogen blistering: the formation of blisters on or below a metal surface from excessive internal hydrogen pressure. (Hydrogen may be formed during cleaning, plating, corrosion, etc.).
Hydrogen damage: a general term for the embrittlement, cracking, blistering, and hydride formation that can occur when hydrogen is present in some metals.
Hydrogen embrittlement: hydrogen-induced cracking or severe loss of ductility caused by the presence of hydrogen in the metal. Hydrogen absorption may occur during electroplating, pickling, or other processes that favor the production of nascent or elemental hydrogen.
Hydrogen overvoltage: overvoltage or overpotenttial associated with the liberation of hydrogen gas.
Hydrogen-induced cracking (HIC): same as hydrogen embrittlement.
Hydraulic cements: hydraulic cements are waterproof and will even set underwater. Such materials are identified by the presence of finely pulverized material added to the cement binder which will cause discoloration from white to pale browns or pinks.
Hydrolysis: a) decomposition or alteration of a chemical substance by water. b) in aqueous solutions of electrolytes, the reactions of cations with water to produce a weak base or of anions to produce a weak acid.
Hydrophilic: having an affinity for water.
Hydrophobic: lacking an affinity for, repelling, or failing to absorb or adsorb water.
Hydrostatic test: a pressure test procedure in which a vessel or system is filled with water, purged of air, sealed, subjected to water pressure, and examined for leaks, distortion and/or mechanical failure.
Hydroxide: a chemical compound containing hydroxyl ion. (see pH)
Hydroxyl: the OH^- anion which has a single negative charge, and provides the characteristics common to bases. (see pH)
Hygroscopic: possessing a marked ability to accelerate the condensation of water vapor; applied to condensation nuclei composed of salts that yield aqueous solutions of a very low equilibrium vapor pressure compared with that of pure water at the same temperature.
Hypochlorite: the OCl^- anion; calcium and sodium hypochlorite are commonly used as bleaches and disinfecting agents.

Link to glossary of corrosion and materials maintenance terms