Environmental protecting coatings
Environments that thermal spray coatings may experience vary in temperature extremes as well and their corrosive nature. The former may be below freezing to red hot; the latter from mildly/highly caustic to mildly/highly acidic.
Environmental protective coating can therefore be separated into two (2) major categories:

· High temperature oxidation and hot corrosion resistance
· Atmospheric corrosion control

High temperature and hot oxidation resistance

All thermal sprayed claddings should exhibit some degree of corrosion resistance. However, there are many applications where the coating is solely intended to offer high temperature oxidation/corrosion protection. Materials are selected based upon their ability to function as a barrier between the corrosive environment and the substrate. Corrosion occurs slowly so the coatings are sacrificed to protect the substrate. It is important that their densities be high so that sealers are not required. In some instances, coating density may be intensified by Hot Isostatic Pressing (HIPping).

Materials for high temperature use include:

· CoCrAlY
· CoNiCrAlY
· FeCrAlY
· NiCrAlY
· NiCoCralY
· Exothermic MCrAlY’s
· Nickel-chromium alloys
· Inco 718
· IN-625
· René 41
·
Atmospheric corrosion control

Flame sprayed coatings of wire aluminum and zinc are the most common thermal spray answers to atmospheric and marine corrosion. Both are anodic to steel.

Zinc provides extends the service life of steel by twenty to thirty (20 to 30) times. Its corrosion products are friable and easily removed thereby presenting an unprotected open surface.

Aluminum protects somewhat differently. Its corrosion product is more tenacious and inert. Internal pores fill with oxide products to prevent the progression of rust. Either coating will provide a steel component with:

· Longer life
· Compatibility with many paints and sealers
· Resistance to mechanical damage
· Resistance to ultraviolet light
· Can usually be applied in-situ
· Do not sag or run
· Can be applied thicker than competitive coatings

Materials for atmospheric and marine protection include:

· Commercially pure aluminum
· Pure zinc
· Zinc-aluminum alloys

Aluminum protects somewhat differently. Its corrosion product is more tenacious and inert. Internal pores fill with oxide products to prevent the progression of rust. Either coating will provide a steel component with:

· Longer life
· Compatibility with many paints and sealers
· Resistance to mechanical damage
· Resistance to ultraviolet light
· Can usually be applied in-situ
· Do not sag or run
· Can be applied thicker than competitive coatings

Materials for atmospheric and marine protection include:

· Commercially pure aluminum
· Pure zinc
· Zinc-aluminum alloys