Aluminized Type 2 Carbon Steel Applications

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

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Government Superior Acceptance

Many federal, state and local government agencies have assigned superior durability ratings to Aluminized Type 2 Steel pipe relative to that of various other pipe materials. The Federal Highway Administration (FHWA) and U.S. Corps of Engineers recommend a conservative 2:1 superiority factor for Aluminized Type 2 over galvanized steel.

All of the states below have designated Aluminized Type 2 as a superior product. Download the product data bulletin for more details:

  • Alaska DOT
  • Arkansas DOT
  • California DOT
  • Connecticut DOT
  • Florida DOT
  • Georgia DOT
  • Idaho DOT
  • Indiana DOT
  • Illinois DOT
  • Iowa DOT
  • Kansas DOT
  • Kentucky Department Of Highways
  • Maine DOT
  • Massachusetts Highway Department
  • Maryland DOT
  • Michigan DOT
  • Minnesota DOT
  • Mississippi State Highway Department
  • Montana DOT
  • Nevada DOT
  • New Mexico DOT
  • New York DOT
  • North Carolina DOT
  • North Dakota DOT
  • Ohio DOT
  • Oklahoma DOT
  • Oregon DOT
  • Pennsylvania DOT
  • South Dakota DOT
  • Tennessee DOT
  • Texas DOT
  • Utah DOT
  • Virginia DOT
  • Washington DOT
  • West Virginia DOT
  • Wisconsin DOT

Updated 2013.

Coating Microstructure

The Aluminized Type 2 steel coating microstructure is comprised of two layers: 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 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's thickness and uniformity assures the degree of coating formability necessary for corrugated steel pipe manufacture.

Coating Corrosion Resistance

Aluminum spontaneously forms a passive oxide layer. 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 layer 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 velocity rainfall run-off carrying bed load 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, providing effective long-term protection. As a consequence of its combined coating properties, Aluminized Type 2 steel 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 that exist in seawater, acid mine water, and sanitary sewage.

Durability Guidelines

Field studies performed on 30, 42 and 50 year old Aluminized Steel Type 2 pipes have established the minimum service life at 75 years for 16 gauge material within the recommended environmental ranges: pH 5 – 9 and resistivity ≥ 1500 ohm•cm. The deepest pitting observed in the studies occurred near these recommended pH/resistivity limits. Worst-case rates for the recommended pH/resistivity ranges project to 104 years for first pit penetration through a 16 gauge pipe wall, supporting the designation of an estimated 100 year service life at 16 gauge through the modified range. Rates for steel substrate penetration calculated from 30 year studies are quite low due to galvanic retardation provided by the coating. The low magnitude of the rates was further confirmed in studies on pipes 42 – 50 years of age.

Fabrication and Corrosion Behavior

In the fabrication of corrugated metal pipe, some mechanical effects of the manufacturing process on the metallic coatings can be expected. The resultant potential impact on corrosion of the substrate must be addressed by the metallic coating. In the case of the Aluminized steel Type 2 coating, the steel substrate is protected in service over the long term due to the electrochemical behavior of the bi-layer, duplex coating. The aluminum layer of the coating provides low-level galvanic protection that directly retards long-term substrate corrosion. This also modifies the corrosion process to produce a partially protective corrosion-product scale that hinders the advance of corrosion. Additionally, substrate corrosion is further retarded due to insulating films that form naturally on the Al and Al-Fe coating layers and suppress the electrochemical action that powers substrate corrosion. All of these electrochemical mechanisms combine to protect against corrosion problems.

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