As a result of their inherent resistance to corrosion, the HAYNES® and HASTELLOY® alloys are generally inert to cold acid pickling solutions. Also, the oxide films that form on these alloys during heat treatment are more adherent than those that form on the stainless steels.
The most effective descaling methods for the HAYNES® and HASTELLOY® alloys are immersion in molten caustic baths, followed by acid pickling at elevated temperatures.
Three descaling methods have been used successfully with the HAYNES® and HASTELLOY® alloys, namely:
- The VIRGO descaling salt bath process.
- The sodium hydride reducing salt bath process.
- The DGS oxidizing salt bath process.
The procedures associated with these methods are shown in the table below.
Descaling and Pickling Procedures
|
|
VIRGO Descaling
Salt Bath
|
Sodium Hydride
Reducing Salt Bath
|
DGS Oxidizing
Salt Bath
|
|
Descaling Bath
|
VIRGO Salt
|
Sodium Hydride
|
DGS Salt
|
|
Bath Temperature
|
970°F (521°C)
|
750°F-800°F (399°C-427°C)
|
850°F-950°F (454°C-510°C)
|
|
Descaling Time
|
1 to 3 minutes
|
15 minutes
|
2 to 10 minutes
|
|
Water Rinse Time
|
1 to 2 minutes
|
1 to 2 minutes
|
1 to 5 minutes
|
|
Pickling Step 1
|
15-17% Sulfuric Acid
+ 0.5-1% Hydrochloric Acid
at 165°F (74°C) for
3 minutes*
|
4-6% Potassium Permanganate
+ 1-2% Sodium Hydroxide
at 135°F-155°F (57°C-68°C)
for 15 minutes*
|
15-25% Nitric Acid
+ 3-5% Hydrofluoric Acid
at 130°F-150°F (54°C-66°C)
for 10 to 20 minutes
|
|
Pickling Step 2
|
7-8% Nitric Acid
+ 3-4% Hydrofluoric Acid
at 125°F-160°F (52°C-71°C)
for 25 minutes
|
8-12% Nitric Acid
+ 2-3% Hydrofluoric Acid
at 125°F-160°F (52°C-71°C)
for 15 minutes
|
No Second Step
|
|
Final Water Rinse
|
3 minutes or Steam Spray
|
Dip
|
Dip and Steam Spray
|
*Followed by a water rinse
Sand, shot, or vapor blasting are acceptable for removing scales, under certain conditions. The blasting materials should be such that they provide a rapid cutting action, rather than smearing the surface. Also, sand should not be re-used, especially if contaminated with iron. After blasting, it is desirable to pickle the work-piece in acid, to remove any embedded iron or other impurities.
Extreme care should be taken when blasting thin-sectioned components with sand, because of the dangers of distortion and/or embedding sand or scale in the metal surface. Sand blasting also tends to work harden the surfaces, which may cause subsequent forming problems for certain alloys.