Aluminized Steel Type 2 was developed in 1939 for superior environmental corrosion resistance. Type 2 combines the strength of a steel substrate with the corrosion resistance of aluminum.

Aluminized Steel Type 2 is produced by continuous hot-dip coating of steel strip in a bath of molten, commercially pure aluminum. Cleaning the strip in a non-oxidizing/reducing furnace atmosphere assures a pristine surface for coating adherence. Interaction of molten aluminum with the steel surface produces a metallurgical bond and provides corrosion protection. Line speed, bath temperature and air finishing knives control aluminum coating thickness.

The Aluminized Steel Type 2 coating microstructure is a duplex one, there being an aluminum layer and an underlying, hard aluminum-iron intermetallic alloy layer. The alloy arises from interaction of molten, commercially pure aluminum with the steel surface and is the agent that bonds the total coating to the substrate. The alloy is an essential part of the coating protection system, supplementing the aluminum layer and providing a second line of defense to ensure long-term durability. Control of the alloy layer thickness and thick-ness uniformity assures the degree of coating formability necessary for corrugated steel pipe manufacture.

CORROSION BEHAVIOR
Coating Corrosion Resistance: The aluminum layer spontaneously forms an aluminum oxide passive film, as is
characteristic of aluminum. This film imparts its usual high resistance to major environmental factors influencing corrosion behavior in waters and soils. Corrosion due to dissolved oxygen and carbon dioxide and erosion corrosion due to high velocity waters are the common influential factors in a pipe waterside environment. The passive film imparts high resistance to all of these factors.

Any pitting in the aluminum layer will be arrested at the thick alloy layer. At the alloy layer, pits grow in width rather than in depth. The aluminum layer may exhibit abrasion losses in high  elocity rainfall run-off carrying bedload but the alloy layer provides enhanced resistance to mild-to-moderate abrasion. The alloy layer also resists erosion corrosion and corrosion by water and soil, thus providing effective long-term protection.

As a consequence of its combined coating properties, Aluminized Steel Type 2 achieves a superior service life over the full range of normal exposure conditions common to drainage pipe environments. Exceptions include severe abrasive conditions and severe corrosive conditions such as exist in seawater, acid minewater and sanitary sewage.

Coating Protection Mechanism of the Steel Substrate: The very slow rate of pit growth observed in field surveys of pipe up to 43 years old is due to the galvanic protection mechanism of Aluminized Steel Type 2. Should pits penetrate into the steel substrate, three combined pit growth-restricting electrochemical factors are activated.

Low-level (partial) galvanic protection of the pit cavity is provided by corrosion of the surrounding aluminum layer.

The electrically insulating oxide films on the aluminum and alloy layers hinder the electrochemical action that powers pitting.

Partial galvanic protection produces adherent corrosion products that plug pit cavities and retard corrosion.

All three electrochemical factors provide effective long-term substrate protection throughout the entire pipe service life.