Link to glossary of corrosion and materials maintenance terms

Corrosion engineering consultant

Corrosion Doctors site map

Alphabetical index of the Corrosion Doctors Web site

Interior Environments - Corrosion

A common misconception is that corrosion may not be a problem for interior steelwork which is protected from the elements. If there is frequent condensation on its surface, corrosion of inadequately protected steel will be significant. Under these conditions, hot dip galvanizing may give more than 40 years life. Hot dip galvanizing has also been used extensively to protect interior steelwork in harsh environments such as swimming pools and breweries. Corrosion may also arise on exposed non-galvanized steelwork intended for interior environments if there is a delay in assembly or construction.

C1: Very Low

Because steelwork in a warm, dry interior will not corrode, the structural stability of a building will not be impaired during it's design life (generally taken as over 50 years). In such conditions, no coating is required.

Inside dry, heated buildings with neutral atmospheres: Offices, shops, schools, hotels, residences, airport terminals, hospitals etc.

C2: Low

When steelwork is exposed to moisture, corrosion will occur at a rate that depends on the severity of the environment.
In low risk environments, systems B2, B3, B4, B9, B10, or B11 are proposed.

Inside buildings where condensation may occur: Exhibition halls, vehicle depots, sports halls etc.

C3: Medium

In medium risk environments, higher-grade systems are needed. B5, B6, B7, B12, B13, B14 are proposed. Inside production buildings with high humidity and some pollution:

C4: High

These guides do not cover high corrosion risk internal environments. Users are advised to seek specialist advice.

Buildings with high humidity and corrosive atmospheres: Chemical processing plant, swimming pools, dye works, paper-manufacturing plants, boat yards over seawater, foundries, smelters etc.

C5: Industrial, Very High

C5M: Marine, Very High

 Notes to tables in interior corrosion guide

  1. In deriving protection systems in the guides, the design life of the structure has been taken as 50 to 60 years. The tables give two figures for durability.
  1. Structure life

    The period of reasonable freedom from severe corrosion of the steelwork that might lead to weakening of the structure. This figure assumes no mechanical damage in service, that no maintenance is carried out and that up to 1mm of steel may be lost from the surface at the corrosion rate for each environment given in ISO 12944. Visible steelwork will normally be accessible for maintenance and if repainting is carried out, the quoted life of the structure will be extended.

  2. Coating life
  1. Costs given are for guidance only. There will be considerable variation, which typically may be +/- 50%, for a variety of reasons. Quotations should be obtained before making the final selection of the protective treatment. The indicative costs given are for 1995. They include estimates for material and labor but exclude taxes. The average surface area per ton is assumed to be 25 m2 ton-1.
  2. 80 mm can be in one coat or as 20 mm prefabrication primer plus 60 mm post fabrication primer.
  3. A "wetting" low viscosity primer is recommended to obtain satisfactory performance.
  4. Alternatively a primer specially formulated for use on fresh galvanized surfaces may be used.
  5. For steel profiles over 6mm thick the minimum average galvanized coating thickness is 85 mm
  6. If painting is required for decorative reasons, any minimal system, which satisfies the decorative requirement, is sufficient.
  7. Where the appearance of the galvanized surface is satisfactory it may be left uncoated but for extended life it would normally be maintenance painted before the zinc layer has disappeared which, according to BS 5493 could be up to 40 years. The subsequent maintenance period will then depend on the paint system chosen.
  8. Where a colored surface is preferred the 10 year figure is for maintenance of the decorative paint coating (which is in addition to the up to 40 year life of the zinc layer).
  9. When a structure durability period exceeding 60 years is required, systems B5, B6 and B7 may be used in environment C2.
  10. Water based coating technology is still developing and advice on durability should be obtained from coating manufacturers.