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Coatings & Linings


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This section attempts to provide a concise review of the most commonly used types of coatings and linings specified for fabricated ductile iron pipe. Through this review we will identify some of the common misconceptions and make recommendations to assist industry professionals with the collective experience of our members who deal with these subjects on a daily basis.

Coatings, discussed first, are differentiated by their basic types, as qualified by our industry’s applications,and by their specific use.

Linings are addressed by definition.

Due to the myriad number of coatings and linings available now, and those being developed for introduction, this review must not be considered to be complete.

 

EXTERIOR COATING

Tar Coating

This is a black coating and commonly applied to the exterior of underground ductile iron pipe. It is properly designated as asphaltic coating.

Bituminous coating refers to similar coatings made from tar pitch. “Bitumastic” a registered manufactures trademark, has been generally used within the industry to refer to bituminous coating. Therefore the term “Bitumastic” without further definition is relatively meaningless.

Both asphaltic and bituminous coatings contains contain oils which may bleed through successively applied coatings. Accordingly, these materials should be removed (re. surface preparation) or sealed with a synthetic resin prior to the application of a finished coating.

Red Primer

Bare exterior pipe surfaces should be painted with a shop applied rust inhibitive primer prior to shipment. Primer should be compatible with the designated final coat. (Re. Surface Preparation).

Coal Tar Epoxy

The most common coal tar coating is coal tar epoxy. Coal tar epoxy is used primarily to protect metal surfaces from rust and corrosion in submerged exposures. Coal Tar epoxy should not be confused with the asphaltic tar normally applied be ductile iron pipe manufactures.

Special Coatings

Due to the complexities of service requirements many specialized coatings are available. These coatings are not covered by ANSI/AWWA Standards.

For further information check with paint manufacturer or your NAPF member.

INTERIOR LININGS

Cement Lining

Cement Lining was first used in cast iron pipe in 1922 with the first standard being adopted in 1939. The primary purpose of cement lining is to provide a smooth flow surface.

An additional benefit is also the protection afforded to the interior pipe surface when subjected to corrosive liquids. See ANSI/AWWA C104 for further details.

Special linings (Heat Applied)

Polyethylene

In this process, a blend of polyethylene powders (ASTM D-1248) and fillers are applied to a pre-heated ductile iron pipe or fittings. Residual heat then melts the powder and fuses it to the interior surfaces forming a smooth flexible film.

This product has been used to provide corrosion resistance against aggressive chemicals wastes.

Since there is no AWWA standard for the application or performance of this product it is generally applied in 40 mils nominal thickness but varies widley depending on the applicators capabilities. Due to the process required to apply this product it has limited availability in sizes smaller than 12" diameter.

Consult your NAPF member for particulars and availability in your region of the country.

UNDERGROUND PROTECTION

The most common form of protection of underground pipe and fittings is through the use of polyethylene film.

It is available in flat sheet and tube form and is used to protect ductile iron pipe and fittings installed underground in hostile soils. This product has been in use for many years and is considered excellent protection.