Field Performance of ALT2 in Montana
Corrugated Steel Pipe (CSP) has a long history of use for culverts and other drainage applications since the product first came to market in 1896.  Galvanized (zinc coated) steel was the first protective metallic coating used to increase the service life of corrugated steel pipe.  Galvanized works well to increase the life span of steel in many environments but is limited in more severe environmental applications.
Many other CSP coatings (both metallic and non-metallic) have been introduced in the market to help increase the life span of CSP.   The two most popular coatings in use today to increase the service life of CSP are Aluminized Steel Type 2 (ALT2), which is a metallic coating and a non-metallic, and TRENCHCOAT® polymer coating. 
The Aluminized Steel Type 2 coating is a two-layer, metallurgically bonded coating.  The ALT2 coating consists of a protective layer of aluminum and an underlying layer of aluminum/iron alloy.  Aluminized coatings are more durable than zinc (galvanized) coatings.  This is due to the formation of a passive aluminum oxide film which is more protective than the scale on zinc.  The film forms on Aluminized in both hard and soft water and is resistant to corrosion by SO4, N3, CO2, and O2, organic acids, and turbulent water.
The Montana DOT installed ALT2 CSP culverts on 28 different sites under State and Interstate highways in the 1980s.  The service life of these Aluminized culvert sites range from 29 – 37 years.  There are three project sites in particular where Aluminized and Galvanized pipes were installed at the same site. 
This summer, twelve (12) of those sites were observed, and the performance was documented.  After several years of in-ground performance, the ALT2 CSP culverts sites appear to be in excellent condition. Consistent with previous industry and governmental agency studies, the ALT2 culverts have provided a far superior life expectancy compared to the galvanized culverts.