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Corrosive environments vs. ventilation systems

corrosion resistant ventilation

Corrosion in ventilation systems

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:

  1. Selection of the right steel grade for the working environment;
  2. Selection of the right thicknesses for the system;
  3. The thickness of a zinc layer;
  4. Correct installation and treatment;
  5. Regular maintenance.

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.

What is galvanizing and its purpose?

The galvanizing process consists in applying a layer of zinc onto easily-corroding metal components.

Galvanizing types

Hot dip galvanizing

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.

Electrolytic (electro-)galvanizing

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.

What sheets can we galvanize?


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.


Coating symbol

Minimum total
coating weight in g/m3
Approximate, typical
coating thickness (µm) 
Z80 80 5,5
Z100 100 7
Z140 140 10
Z180 180 13
Z200 200 14
Z225 225 16
Z275 275 20
Z350 350 25
Z450 450 32
Z600 600 42


The coating thickness for individual coating symbols.

 

Why does galvanizing protect steel from damage?


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.

Corroded ductwork


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.

Why can you often see “white rust” on galvanized steel?


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.

Corrosivity categories according to PN-EN ISO 12944-2:2001


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: 2017-08EAtmospheric corrosion load

 

Annual loss of zinc coating thickness in µm×a-1

IndoorOutdoor
C1
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
C2
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
C5

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
CX
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

MetalCorrosive
category

Service life (years)

125101520
Carbon steel C1 1,3 1,9 3,0 4,3 5,4 6,2
C2 25 36 58 83 103 120
C3 50 72 116 167 206 240
C4 80 115 186 267 330 383
C5 200 287 464 667 824 958
CX 700 1006 1624 2334 2885 3354
Zinc C1 0,1 0,2 0,4 0,6 0,9 1,1
C2 0,7 1,2 2,6 4,5 6,3 8,0
C3 2,1 3,7 7,8 13,6 19,0 24,0
C4 4,2 7,4 15,5 27,3 38,0 48,0
C5 8,4 14,3 31,1 54,6 75,9 95,9
CX 25 44 93 162 226 286


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].

Discover the advantages of galvanizing

 

  • An appealing surface finish that keeps the appearance attractive for a long time.
  • Better protection against discolouration and rust.
  • Protection from mechanical deformations, abrasion and quality degradation.
  • Resistance to the action of organic and inorganic substances allowing long service when exposed to the weather.
  • Proven reduction of material and financial losses caused by corrosion.
  • Post-service galvanized steel is a material that can be reused as a raw material in the processing industry.

 

Looking for more information about ventilation systems for industrial purposes? Read our expert articles:

  1. How-to: Welding plastic ventilation ductwork
  2. 8 Common Dust Extraction System Mistakes
  3. Ventilation systems for swimming pools
  4. Modular Dust Extraction Ducting
  5. Plastics in ventilation
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