Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
Materials of Construction - Alonizing/Calorizing
January 10, 2011
The addition of aluminum to all grades of carbon and stainless steels is known to improve their corrosion resistance. However, the addition of enough aluminum to effectively make this improvement usually results in unfavorable changes in the mechanical properties of the base steels.
Alonizing® or Calorizing is the solution to the problem. The Alonizing® or Calorizing process diffuses aluminum into the steel to form an alloy with excellent heat and corrosion-resistance properties; yet the base steel retains its inherent strength and rigidity. Alonizing®(1) does not change the high-temperature mechanical properties of the base steel. Consequently, it is these high-temperature values that will determine the specification of the steel to be Alonized. The protection provided by the Alonized diffusion zone remains effective at all temperatures up to the melting point of the base metal.
Alonizing® does not produce an aluminum coating on the steel substrate. Instead, the process forms a true alloy with the base material. The protective diffusion zone therefore cannot be removed except by a machining operation. The nature of the process, the air-tight retorts, and the controlled atmospheres assure uniform alloy protection over the entire surface of the alonized materials.
(1) The remainder of the description will be done in terms of the Alonizing process. The Calorizing process is similar.
The Alonizing® process is accomplished by positioning the steel in a retort and surrounding it with a mixture of blended aluminum powders. The retort then is hermetically sealed and placed in an atmosphere-controlled furnace. At elevated temperatures, the aluminum deeply diffuses into the steel to form an alloy with the steel substrate. After furnace cooling, the steel is taken out of the retort and excess powder is removed. Secondary operations such as straightening, beveling, trimming, etc are then performed as required.
In general, Alonizing® will not resist the attack of those chemicals and gases which corrode aluminum. Alonizing® should not be used where the temperature is low enough where condensation from the gas will occur.
Welding of Alonized material is possible sine there is no tendency for the surface alloy to break down, melt or volatilize in the areas adjoining the weld. The weld itself must posses the necessary corrosion resistance to the process medium.
An Alonized heat exchanger tube will occupy a slightly larger volume than the un-Alonized substrate and will be up to 0.020" greater in diameter than the original tube. Tubesheet holes should be drilled larger than those recommended by TEMA standards in order to accomodate and permit easy passage of the larger diameter tubes. Alonized tubes are generally rolled rather welded into the tubesheet. It is important that free expansion of the tube beyond the tubesheet is avoided. Belling or flaring of the tube ends is not recommended. The rolling techniques should be one that utilizes automatic control of the tube expansion rather than depending on th skill of the operator for consistent results.
It has been demonstrated that scaling of 304SS converters can be prevented by calorizing the material. One plant owner whose converter experiences severe scaling is proceeding to calorize replacement converter 304SS internals for replacement. The replacement parts include support grids and posts are fabricated and then calorized. The company performing the calorizing is Quantum Ceramalloy.
|Grids delivered from fabricator to be calorized|
|Cross-section of calorized test piece showing aluminum layer|