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Material Selection and Corrosion Resistance

Best Practices

Corrosion of metallic materials is a common concern in most applications. It is very important that the environmental conditions of a project be reviewed and understood thoroughly before selecting the proper material.  Proper material selection will ensure that the cable tray products meet the expected service life without a reduction in performance.
Cable trays are mainly exposed to atmospheric corrosion.  The environment in which the cable trays are installed is therefore the main criterion in the choice of surface treatment or the type of steel.  Cablofil offers wire mesh cable tray in four basic material/finish options.  The following chart is an estimate of the typical service life of the four basic environments.

  .

 

Environment and Service Life Guidelines

 

Material 

Finish

 Typical Environment/Application

End-of-Life Expectation 

Carbon Steel ASTM A510 Grade 1008 EZ - (ASTM B633)Electrogalvanized Steel INDOOR \ Commercial Office, Retail, Institutional, Computer Room, Light Industrial, Warehousing Less than 5% red rust in most dry environments after 20 years.
Carbon Steel ASTM A510 Grade 1008 GC - (ASTM A123) Hot-Dip Galvanized Steel OUTDOOR \ Power Plants, Industrial Less than 5% red rust in most outdoor environments after 30 years.
AISI Type 304L 304L – Type 304L Stainless Steel INDOOR-OUTDOOR corrosive\ Wash Down, Industrial

Less than 5% rust stain in most outdoor environments after 30 years. No significant change to base material.

AISI Type 316L 316L – Type 316L Stainless Steel OUTDOOR - Highly corrosive \  Chemical, Offshore, Marine, Waste Water, Pulp and Paper Less than 5% rust stain in most outdoor environments after 50 years. No significant change to base material.

 

Zinc coated steel products
Galvanic protection of steel is a sacrificial process.   Zinc, in contact with the oxidizing agent, is converted into zinc hydroxycarbonate (white) before the steel will oxidize, ie rust, therefore protecting the steel.  Dry or mild wet/dry environmental cycles will cause the zinc to oxidize at a rate slow enough to remove the hydroxycarbonate (white) residue.  When zinc is left in constant contact with the oxidizing agent, the white zinc residue will build up at an accelerated rate and will not be removed.

EZ Electrogalvanizing after manufacture

A coating of zinc is deposited on to steel by electrolysis. The baths used consist of acid or alkaline solutions of zinc salts; the anodes are zinc (generally 99.99% pure). The parts to be coated, degreased and cleaned constitute the cathode.

 

ASTMB 633 - Standard EN 12 329

1. Degreasing for 20 minutes at 50°C, and cleaned twice with water. 2. Stripping in hydrochloric acid at normal temperature. 3. Dipping in an alkaline or acid zinc bath.

4. Acidic zinc coating: cable tray is connected to cathode; balls of zinc are in a basket connected to anode. Time of operation: 25 minutes at 40°C. 5. Passivation: Chemical reaction of zinc by immersion, which gives the blue aspect of the finish. 6. Drying

 

The cable tray, manufactured using untreated steel wire, is pickled and then immersed in an electrolyte containing zinc. Zinc is deposited on it by passing an electric current.

 

A smooth, typically bluish-grey and glossy appearance is obtained depending on the pH value of the electrolytic bath used. The colour and gloss have no negative or positive effect on the corrosion resistance of the coating.

 

This type of coating is primarily suited for indoor installations.

Concerned about the zinc whisker phenomenon?  Click here for more information.


Hot dipped galvanized after manufacture

After having undergone rigorous chemical preparation, the steel is immersed in fused zinc (450 to 460°C). Several layers of alloy are formed deep within the metal and attach to the zinc on the surface. (White or grey zones of zinc oxide, due to the formation of zinc hydrocarbonate, can appear on the surface without affecting resistance to corrosion.)

 

ASTM A123 - Standard EN ISO 14 61 (formally BS EN 729)

1. Cleaning the tray to remove dirt and any existing rust.

2. Immersing the tray in a bath of zinc at 450°C.

3. The chemical reaction takes place. Layers of zinc are formed.

4. The tray is cooled, passivated and inspected by the Quality Department.

 

The cable trays or accessories manufactured from untreated steel sheet or wire are degreased and pickled before being immersed in a bath of molten zinc. The whole part is therefore covered with a thick layer of zinc.

 

A light grey, slightly rough appearance is obtained.

 

This relatively expensive process is essential to withstand highly corrosive atmospheres.

 

Note: Any white marks due to the formation of zinc hydroxycarbonate which might appear on the surface have no influence on the corrosion resistance. This is in fact the principle on which galvanic protection is based.


Stainless Steel (SS) Finish

There are numerous types of stainless steel: austenitic, ferritic, martensitic. Cablofil® has selected 2 austenitic steels, 304 L and 316 L, for their excellent corrosion resistance.  This is primarily due to their very low carbon content ("L" for "low carbon").

 

Aggressive environment: Stainless Steel

In highly aggressive environments, rather than changing the type of surface treatment, a different type of steel is used. With its wide Stainless steel range, Cablofil® can be installed everywhere, even under the harshest conditions.

 

Damage by abrasion, cutting or machining is quickly repaired.

 

 

 

Fig. 1 - In any normal oxidizing environment, a protective coating of passive, chromium-rich oxide film is automatically formed on stainless steel.

Fig. 2 - When scratched, damaged or machined, this protective film is denuded. The steel is exposed to the atmosphere.

Fig. 3 - The protective coating is quickly restored through the rapid self-repairing quality of the chromium-rich film.

 

Decontamination of Stainless Steel

Post-treatment of stainless steel is as important as the choice of grade. The manufacturing operations (straightening, welding, profiling, stamping) pollute the stainless steel with particles of carbon steel, which affect the product quality.

  • Acid pickling, before degreasing, eliminates these pollutants.
  • Passivation consists of artificially creating a film of chromium oxide on the surface thereby creating a uniform finish and giving the stainless steel optimum corrosion resistance.

These two steps, considered as crucial by Cablofil®, guarantee product lifetime and therefore the lifetime of the installation. The salt spray and SO2 (sulphur dioxide) corrosion resistance tests demonstrate this point.

 without decontamination

 with decontamination

 

 

 

Pickling and passivation give the Cablofil® stainless steel a very light grey and matte finish. The correct choice of stainless steel grade, followed by pickling and passivation, ensures that Cablofil® cable tray can withstand marine, chemical and agriculture environments.

Stainless Steel 304L

Standard EN 10088-2 - AISI 304L - X2CrNi18.09.

Mainly used in the food, chemical and petrochemical industries, this stainless steel offers excellent corrosion resistance in most environments.

304L Stainless Steel

Excellent corrosion resistance in a wide range of media, such as:

  • Freshwater
  • Natural Atmospheres/Organic Chemicals
  • Most Food Processing Environments
  • Most Architectural Applications
  • Dyes
  • A wide variety of Inorganic Chemicals
  • Can be readily cleaned
  • Not vulnerable to intergranular corrosion.

 

(304L SS is successful in warm chloride environments where exposure is intermittent and if it can be regularly cleaned.)

Stainless Steel 316L

Standard EN 10088-2 - AISI 316L - X2CrNiMo17.12.2 

Since it contains molybdenum, stainless steel 316L offers almost perfect corrosion resistance in all types of industry, especially in the highly aggressive halogen-containing environments (fluorine and chlorine).

316L Stainless Steel

  • Very low carbon content, with the presence of molybdenum
  • Designed particularly for:
  • Chemical and Food Industries
  • Nitrate explosive industry
  • Decorative Treatment Workshops
  • Photographic Laboratories
  • Not liable to intergranular corrosion.
  • Better resistance to pitting corrosion than the other chromium-nickel stainless steels, where brines, sulphur-bearing water, or halogen salts (such as chlorides) are present.
  • Valuable property: High creep strength at elevated temperatures.

 

(Other mechanical properties/fabricating characteristics are similar to 304.)


Epoxy coated or Custom Painted

Resin-based paint is applied on the cable tray using an electrostatic powder process then cured in an oven. The entire range of RAL colours can be obtained.

Mainly used for aesthetic reasons, epoxy paints offer very good corrosion resistance.

 

 


 

  

PG/GS: continuous galvanisation before manufacture using the Sendzimir process (pre-galvanised)

Standard GS (accessories): EN 10142
Standard PG (wire): EN 10244-2

Before manufacture, a coating of zinc is deposited by continuous immersion on the steel sheets or wires. The appearance of the components is now smooth and grey.

This coating is suitable for only slightly aggressive atmospheres, ideally indoors.


Dacromet®

The process involves three stages: surface preparation, application of the treatment and finally curing. A coating based on zinc and aluminium is obtained, giving the parts a smooth, light grey appearance with no grain.

Dacromet® offers protection equivalent to that of GC and is only used for small accessories and fixings, which are difficult to hot dip galvanize.

   

Best Practice

Galvanic corrosion
Galvanic corrosion is the result of an electrochemical phenomenon due to the potential difference between different metals, or between a metal and the impurities it contains, when they are in electrical contact.
It is necessary to take into account this phenomenon when selecting the supports, fixings and connections. However the results listed below are based on laboratory conditions and testing.  Real work installation requires other conditions to be taken into affect to determine if significant galvanic reactions will occur.

 Cable tray

 Accessories

EZ/PG
GC
304 L
316 L

EZ/GS
GC/DC
316 L
316 L

 

Conditions

Lab Tests Typical Cable Tray installation
1. Submerged in Seawater Wet/dry cycles not constant immersion
2. Equal mass materials Primary material may be 100 times greater
3. Great connection Electrical current/connector


Recommendation for Typical Cable Tray areas

Tray material/finish   Hardware Finish    
 

Zinc-Plated

Dacromet

GC

SS316

Steel/EZ (Electro-zinc)

R

R

R

A

Steel/GC (HDGAF)

N/R

R

R

R

Steel/BL (Painted)

R

R

R

R

Steel/PE (Painted)

R

R

R

R

Stainless-steel 304 (passive)

N/R

A

A

R

Stainless-steel 316 (passive)

N/R

A

A

R

Aluminum

A

R

N/R

R

(R:Recommended, A: Acceptable, N/R: Not Recommended)

Reference: American Galvanizers Association (Ask Dr.Galv)