Glossary of Terms

 

Term

Definition

Air lock An area where air is trapped in a section of an article and prevents the molten zinc from contacting the steel’s surface, causing an uncoated area on the work
Aluminium-killed steel Steel produced with aluminium used as the deoxidising agent.  This steel will typically have a low silicon content
Ash Zinc oxidation products formed on the surface of the galvanizing bath as the molten zinc reacts with oxygen in the air.  This ash is skimmed off the surface and recycled
Bare spots Uncoated or ungalvanized areas on the steel surface
Bimetallic corrosion The corrosion that occurs when two different metals are in direct contact with an electrolyte (moisture) present.  The more anodic metal corrodes preferentially to the other metal. Also known as ‘galvanic corrosion’.  Zinc will always corrode in preference to the steel – it protects the steel
Centrifuging Spinning of articles, generally small and loose in a basket, after withdrawal from the molten zinc to assist drainage and remove excess zinc
Coating mass The combined mass of zinc and/or zinc alloys formed after galvanizing on the surface of the steel, expressed in grams per square metre (g/m2)
Coating thickness The combined thickness of zinc and/or zinc alloys formed after galvanizing on the surface of the steel, typically derived from an average of thickness measurements and expressed in micrometres or commonly microns (μm).  Note: The American term (mils) is not the same as microns and 1 mil = 25.4 μm
Corrosion rate The amount of corrosion that occurs in a given time. Usually measured in micrometres (μm) per year.
Corrosivity A measure of the ability of the environment to cause corrosion
Degreasing The first stage in the pre-treatment of the hot dip galvanizing process which aims to remove organic contaminants e.g. oils and paints, from the steel surface.  This is usually carried out with caustic
Design life The period over which a structure or structural element is required to perform its intended purpose with appropriate maintenance
Dimensional instability (distortion) Dimensional change, deformation, warping or buckling of articles
Double dipping When articles/items are longer or wider than the galvanizing bath in one dimension and galvanized in two dips, where one section of the article is dipped, withdrawn and then the article rotated to allow the second section to be dipped. A thicker coating cannot be achieved by double dipping
Dross inclusions Dross is small zinc-iron crystals in the galvanizing bath formed from iron reacting with molten zinc.  Dross is heavier than zinc and settles to the bottom of the kettle where it is periodically removed. If the dross is stirred up and mixed into the body of the molten zinc it can become trapped in the coating leaving marks like pimples on the surface.  An excessive amount of these can indicate a poor coating
Duplex coating When galvanized surfaces are painted, or powder coated, the combined coating system is called a duplex coating. Duplex coatings have a synergistic effect, and the life of the combined system is greater than the sum of each component
Durability The time elapsed before the first major maintenance necessary to arrest corrosion
Fettling Smoothing, cleaning or conditioning of the hot dip galvanized coating after dipping to ensure function and appearance requirements are met which may include removal of sharp edges, lumps and repair
Fluxing The last stage of pre-treatment in the hot dip galvanizing process, where a pre-flux solution, zinc ammonium chloride, is used to activate the article’s surface prior to immersion in molten zinc
Galvanizing The process of dipping a prepared article into molten zinc, resulting in the production of a hot dip galvanized coating on a steel article
Jig A fixture used to hold or hang articles from in a particular orientation, facilitating handling of articles throughout the galvanizing process, usually by crane
Life to first maintenance The time interval that can elapse after initial coating before coating deterioration reaches the point when major maintenance is necessary to restore protection of the steel
Macro-environment Refers to the long-term climatic conditions for a particular area (e.g. rainfall, temperature and relative humidity, the levels of airborne salinity and pollution levels)
Masking The technique of covering areas with a substance which will not allow the covered area to be galvanized
Micro-environment Refers to the local conditions (e.g. natural topography, the built environment or vegetation) and site conditions (e.g. water ponding, sheltering of the structure from local rain or other building materials in contact with the structure) which can create very different local effects compared to the macro environment
Patina The dull grey passive film, mostly consisting of zinc carbonate, formed on the hot dip galvanized surface over time via a series of reactions with oxygen, moisture and carbon dioxide
Pickling The second stage of pre-treatment in the HDG process involving immersion of the steel in an acid solution to remove rust and mill scale from the steel’s surface
Quenching Cooling of the article after withdrawal from the molten zinc by immersion in water and/or a low concentration sodium dichromate solution
Reactive steel Steel which has an increased reactivity with molten zinc due to the chemical composition, particularly silicon and/or phosphorous content
Service Life The time (generally expressed in years) from initial coating to the point where first major maintenance of the coating is first required
Significant surface The part of the article for which the coating is essential for serviceability and/or appearance
Silicon-killed steel Steel produced with silicon used as the deoxidising agent
Sweep blasting Light abrasive blasting to clean and provide a surface profile for the hot dip galvanized coating prior to application of paint or powder coating.  Refer to Clause 7 of AS/NZS 2312.2.  Equivalent to SSPC SP-16 ‘brush-off blast cleaning of coated and uncoated galvanized steel, stainless steels, and non-ferrous metals’
Venting The allowance for air or moisture to escape from all features of an article, especially hollow sections, and the pre-treatment and molten zinc to contact all surfaces during immersion
Weld seepage The emission of previously retained pre-treatment solutions from narrow spaces between two closely contacting surfaces that have been subject to intermittent welding or from very small cavities (pinholes) in the welds of a galvanized article
Weld spatter Oxidised, normally spherical, expelled weld metal that can fuse to the surrounding material during welding, generally found adjacent to weld areas.  Should always be removed by the fabricator prior to galvanizing.
Wet storage stain or White rust A white, generally powder-like, substance although can be in the form of black spots, which generally consists of a mixture of zinc oxide and zinc hydroxide; the natural corrosion products of zinc
Zinc-iron alloy(s) Crystalline formation of zinc and iron resulting from a diffusion reaction of molten zinc with iron. A typical hot dip galvanized coating contains three layers with different ratios of zinc and iron