Internal steel coatings – are they necessary?

Protective coatings are often applied to external structural steel to prevent its exposure to weather and moisture, which you can read about here. But what about internal structural steel?

There are many standards and references for choosing a structural steel coating, which is to be expected when there are so many considerations.

Here, we discuss coating considerations for internal or enclosed steel.

Please note you should always discuss your specific situation with your certifier or a Certified Practicing Structural Engineer (CPEng).

This article is not advice but merely a discussion of the information in the steel industry and standards about internal structural steel.

How to choose the right steel coating for the job

Edcon Steel-Internal or enclosed structural — *Figure 2*

What is internal or enclosed structural steel?

The term fully-enclosed steel, or enclosed steel, has been used in the steel industry for decades. Manufacturers define it as structural steel that is sealed within the building envelope, such as air-conditioned buildings and sealed houses. Loosely speaking, the steel used in the internal structure of a house or domestic building is considered enclosed.

The degree of enclosure of the supporting steelwork must then be considered; if structural steel is enclosed by masonry walls (which include provision for sub-floor ventilation) with a maximum size/area of openings as shown in Figure 2, it is considered enclosed.

The important point to make here is it is not in contact with the external cavity of the building.

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What should you do to protect enclosed steel?

Well, the short answer is…NOTHING!

Save your money, and your time, by using the steel as it comes from the mill (mill finish).
If you’re not convinced, let’s dig a bit deeper.Firstly, the recommendation from the Australian steel manufacturers is that no further treatment is required to be applied to enclosed steel.

The Australian/New Zealand Standard AS/NZS 2312, “Guide to the protection of steel against atmospheric corrosion by the use of protective Coatings” (the Standard), does not specifically mention enclosed steel.

The lowest corrosivity category in the Standard is called “C1: Very Low,” and this applies to the insides of shops and offices.

Because enclosed steel is too low on the corrosivity measure, we can check the Australian/New Zealand Standard, Guide to the Painting of Buildings (AS/NZS2311), which states:

“Where no specific application standards apply, the minimum standard to follow is the recommended specification from the relevant manufacturer specific to the product or type of substrate”.

In layman’s terms: refer to the manufacturer’s recommendations. These clearly state that no further protection is required for enclosed steel.
Still not convinced?

The National Construction Code (2019) corroborates the steel industry’s (and our) interpretation of the Standard, with clause 3.4.4.4 recommending no treatment of internal steel in a moderate environment. In the more recent 2022 National Construction Code, the protection of steel is covered in section 6.3.9. Table 6.3.9a starts with the environment called LOW (similarly referenced in the AS/NZS2312 as Corrosivity Category “C2: Low”), a category that includes warehouses and sports halls – this is clearly not referring to enclosed steel.

As LOW is now the lowest case considered by the NCC2022 and clearly does not refer to VERY LOW (or enclosed steel), we can only assume that the NCC leaves it to others to concern themselves with protective coating systems in these environments.

What are the exceptions?

Like everything, there are always exceptions, and you should make sure you discuss the specifics of your project with a certifier to ensure you have covered your bases and that they will sign off on your structural steel.

If you have an engineering specification, then you must follow this to the letter.

As we noted above, enclosed steel does not include steelwork that is in contact with the external cavity of the building or partially enclosed steel.

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