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High Temperature Corrosion - Why Do This Happen to my Equipment?
Rodger Seeley - Hendrickson International Corp.

First let us define "high temperature corrosion". It is anything that degrades equipment above 1000F.

We can illustrate "high temperature corrosion" as a football play in schematic, where the "environment team" is trying to invade the "metal" team's territory by penetrating the "surface" or scrimmage line. We can control the outcome of this by focusing our attention on the "magic triangle" of conditions: namely A) "Chemistry of the environment", B) "Time and Temperature of exposure", and C) "Material used for the Equipment".

Let's consider the common types of High Temperature Corrosion.
Oxidation:
       Iron Oxide Formation
       Nickel Oxide Formation
       Cobalt Oxide Formation
       Copper Oxide Formation
       Aluminum Oxide Formation
       Silicon Oxide Formation

These have been arranged in order of low to high free energies of formation, or less stable to most stable in the oxide form.

Sulfidation:
Sulfidation occurs in reducing, low oxygen environments. Sulfides are not strong and flake off their metal hosts easily. This form of corrosion blocks the formation of the more stable oxides, ties up chromium, and creates chromium depleted areas which then suffer from excessive oxidation corrosion.

Carburization:
Carburization occurs when there are high carbon gasses in the environment with high carbon potential. Most metal carbides form at grain boundries and cause low temperature brittleness. Interestingly at high temperatures these carbides are as ductile, and sometime stronger than the host metal without the carbides. The extreme form of carburization corrosion is referred to as "metal dusting". Smooth cavities are left as a result of such dusting.

Nitriding:
Nitrogen can penetrate metal surfaces and form nitrides in grain boundries.

Corrosion at solid metal/molten metal interfaces
In this situation solubility is the key corrosion property. For example Nickel is very soluble in Lithium.

Corrosion by molten salts:
In this situation, fluxes erode away oxides which otherwise might be protecting the equipment.

Ash and salt deposites:
These sorts of thing are very prevalent in air burning gas turbines (particularly along the seacoast), and also in incinerators.

Industries Effected by HT Corrosion

Heat treating equipment suffers damage to muffles, tubes, pots, fixtures, and wire belts.
Chemical Processing Industry Equipment
Waste Management equipment such as incinerators, superheaters, Thermalcouple protection tubes, and soil remediation equipment.
Power generation equipment
Refinery equipment
Mining and Extraction Equipment

Knowing if you are being attached by HT Corrosion:
       Watch for equipment break downs and analyze the failed components.
       Monitor the appearance of high temp equipment and possibly thicknesses.
       Pay attention to degrated process performance of the high temperature processes

Two case studies were presented both involving sulfidation attack and both to Calciners. The basic approaches used are SEM/EDAX evaluation of failed components. Complete analysis of the environment (including process materials) is also beneficial in understanding the High Temperature Corrosion.

Avoiding High Temperature Corrosion
Select optimum materials
       Ask Producers
       Learn from Technical Papers (ASM, TMS, NACE)
       Ask other processors
       Read references and visit web sites such as: www.haynesintl.com, www.smcwv.com, www.sandvik.com, www.cartech.com, www.kruppvdm.com.

Predict the evnironment
       Stability diagrams are available for various environments and temperatures

Predict the Equipment
       Time and temperature effects
       Constant temperature corrosion charts
       Constant time temperature cycle charts
       Break away corrosion curves
       Metal effected rates (loss + penetration

Check out candidate material
       Place coupons into process at critical locations
       Examine by comparison

Protect the surface
       weld overlay of less corrosive materials
       Flame Spray Thermal barrier coatings
       Bonding of protective layer (explosive, roll, or co-extrusion bonding)

Keep the process under control
       Fuel Quality        Feedstock Chemistry        Temperture control

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