Pipes and ducts in HVAC systems are important for the operation of residential buildings as well as industrial facilities. The corrosion resistance and strength of the steel ventilation system depend mainly on such factors as:
Undesirable changes in the structure of the system material are largely dependent on the thickness of zinc coating. The following article explains what the galvanizing process is and what changes take place in the material during service.
Today, galvanization is one of the most effective methods of protecting metal components against corrosion. There are different types of galvanization. Each of them has its pros and cons.
The galvanizing process consists in applying a layer of zinc onto easily-corroding metal components.
The process of applying a mechanically strong zinc layer to steel by immersing steel products in a liquid zinc bath after first preparing them by degreasing and pickling.
a process based on electrolysis, in which electric current is the source of the necessary voltage. The surface of the galvanized components must be thoroughly degreased beforehand and then pickled. These operations remove corrosion. Only after this preparation the material is electro-galvanized.
Any type of steel that meets the requirements of DIN 17100, PN-88/H-84020 and PN-86/H-84018 can be galvanized. The carbon (C) and silicon (Si) content of hot-dip galvanized steel should not exceed 0,5% in total.
The galvanizing process is the most effective if the steel contains less than 0.03% of silicon. If the steel contains between 0.12% and 0.3% of silicon (PN-EN 10025 standard), the quality of zinc coatings is worse (their thickness, gloss, smoothness, adhesion).
The correctly selected zinc coating thickness will protect systems from corrosion. Components without anti-corrosion protection or with an incorrectly applied or damaged coating may start to rust within a very short time. According to PN-EN 10346:2009, the approximate zinc coating thickness is measured in micrometers (µm) or expressed in mass units, i.e. in g/m2.
|Minimum total |
coating weight in g/m3
|Approximate, typical |
coating thickness (µm)
The coating thickness for individual coating symbols.
Ability of zinc to provide cathodic protection. The operating principle behind this process is the difference of potentials. According to the metal electromechanical series, the standard zinc potential is − 0.76 V and is more electronegative than the iron potential of − 0.45 V. Metals with lower potentials have reduction capacity towards metals with higher potentials.
Under the influence of weather and moisture (naturally present in the air) as well as a potential difference between these metals, a spontaneous chemical phenomenon occurs which causes current to flow and zinc to oxidate (corrode). The iron in steel attracts zinc electrons, which effectively stops the corrosive destruction of the steel core of a component.
Example of corroded ductwork caused by wrong selection of duct material.
Zinc is slowly dissolved as a result of the action of the zinc-water-steel electrochemical cell, thus providing cathodic protection to steel (as a result of the flowing electric current) where the zinc coating is broken. As a result of this process, rusty spots do not form on the surface of galvanized steel during the first exposure.
During service, a passive film of zinc oxide and carbonate is created on the zinc surface, which is tight and resistant to corrosion in the circulating water environment, in evaporative coolers and spray-evaporative condensers.
When the zinc coating is exposed to air, it oxidizes and compounds such as zinc oxide, zinc hydroxide, zinc hydroxide carbonate or hydrated compounds containing sulfates form. In particular, zinc oxide and hydroxide contribute to the formation of a colored deposit layer called “white rust”.
It constitutes a natural layer preventing further occurrence of deeper pitting corrosion. Due to the electrochemical properties of zinc, even if the zinc coating is slightly damaged, the steel is still protected. A layer of patina will naturally form at the damaged place.
The purpose of the zinc coating is also to protect the material from mechanical damage. Thanks to its layered structure, it is of various hardness at various depths, which makes it resistant to damage, abrasion, scratches and cracks. The coating produced by this process is permanently fused with steel, as zinc atoms penetrate into it forming a uniform, inseparable alloy.
The table below presents the annual zinc loss in individual corrosivity categories per year. Manufacturers of products cannot determine the corrosivity category which products are fit for. It is the system designer who must determine the environment in which a given component will be used and then assigns it to the corrosivity category.
The next step is to calculate the estimated number of years that the material will last in this environment. A product may be used in line with the corrosivity category determined by the user according to PN-EN-ISO-14713-1_2017-08E, and the coating will be lost in the first year at the rate specified in Table No. 1. In later years of service, the loss of the anti-corrosion coating should be estimated based on PN-EN-ISO-9224_2012E; the values of losses (coating thickness) are presented in Table No. 2.
|Corrosivity category according to PN- EN ISO 14713-1: 2017-08E||Atmospheric corrosion load||
Annual loss of zinc coating thickness in µm×a-1
Heated spaces with low relative humidity and slight pollution, e.g. offices, schools, museums.
|Dry or cold environments, with very low pollution and humidity, e.g. desert.||≤0.1|
Unheated spaces with different temperature and relative humidity. Low condensation frequency and low pollution, e.g. warehouses, sports halls.
|Moderate zone, atmospheric environment with low pollution of SO2 <5 µg/m3, e.g. rural areas, small cities.||> 0.1 to 0.7|
|C3||Areas of moderate condensation frequency and moderate pollution from production processes, e.g. food processing plants, laundries, breweries, dairies.||
Moderate zone, atmospheric environment with medium pollution (SO2: 5 µg/m3 to 30 µg/m3) or with chlorides, e.g. urban areas, coastal areas with low chloride deposition. Subtropical and tropical zones with low pollution.
|> 0.7 to 2.1|
|C4||Areas with high pollution from production processes, e.g. processing plants, swimming pools||
Moderate zone, atmospheric environment with high pollution (SO2: 30 µg/m3 to 90 µg/m3), significant impact of chlorides, e.g. polluted urban area, industrial areas, coastal areas, access to salt water, exposure to strong salt action. Subtropical and tropical zones with moderately polluted atmosphere
|> 2.1 to 4.2|
Areas with a very high condensation frequency and/or high pollution from the production processes, e.g. mines, caves, industrial cubicles, non-ventilated shelters in tropical and subtropical zones
|Moderate and subtropical zones, atmospheric environment with a very high level of pollution (SO2: 90 g/m3 to 250 g/m3) and/or a significant share of chlorides, e.g. industrial areas, coastal areas||> 4.2 to 8.4|
Areas with almost constant condensation or long periods of exposure to extreme humidity and/or with high pollution from the production processes, e.g. unventilated shelters in humid tropical zones with the penetration of external pollutants, including chloride molecules and corrosive solid particles
Subtropical and tropical zones (frequent presence of moisture), atmospheric environment with a very high pollution (SO2) (exceeding 250 µg/m3), including factors accompanying production activities, strong impact of chlorides, e.g. extreme industrial areas, coastal areas, salt mist areas
|> 8.4 to 25|
Table No. 1. Description of typical atmospheric environments with their estimated corrosivity categories
according to PN-EN ISO 14713-1:2017-08E
For C4 and C5 environments where corrosion risks are high, stainless steel sheet - such as 1.4301 / 304 and 1.4404 / 316L - is especially recommended.
Service life (years)
Table No. 2. Maximum corrosion loss during longer exposures for different corrosivity categories acc. to PN-EN-ISO-9224_2012E. Material loss values are presented in micrometers [µm].
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