25 May 2005
however, for most applications, a 10% solution is adequate. The following formulation is
for a 10% hydrochloric acid solution. It can be used for removing scale consisting
primarily of carbonates with lesser amounts of phosphates, sulfates, and silicates. This
type of scale is typically found in a steam boiler system containing copper alloys that
has been treated with a phosphate-based program. Depending on the specific descaling
application, some of these ingredients can be omitted from the formulation. For
example, diethylthiourea is not needed if there is no copper in the system. It should be
noted that if diethylthiourea is used, the waste material should be treated as a
hazardous waste. Where there is only carbonate scale to be removed, ammonium
bifluoride, which is used to remove silica-based scales, may be omitted. The addition of
a wetting agent is preferable but not absolutely necessary.
Example Procedure for 10% Solution. The following is an example
procedure that can be used to make 3785 liters (1000 gallons) of a 10% solution:
1. Add 1079 liters (285 gallons) concentrated (36% strength) hydrochloric
acid, American Society for Testing and Materials (ASTM) E 1146,
Specification for Muriatic Acid (Technical Grade Hydrochloric Acid), to
approximately 2271 liters (600 gallons) of water.
2. Add the proper amount of a corrosion inhibitor, Military Specification MIL-I-
17433, Inhibitor, Hydrochloric Acid, Descaling and Pickling, recommended
by the manufacturer to the diluted acid solution. The inhibitor must be
compatible with hydrochloric acid and must not precipitate under any
condition during the cleaning operation.
3. In a separate tank containing about 284 liters (75 gallons) of water:
a) Add 39 kilograms (85 pounds) of the chemical
(1,3) diethylthiourea to complex any copper and keep it from
depositing. Do not use the diethylthiourea as the corrosion inhibitor
required in paragraph 9-2.2.1(step 2) above.
b) Add 55 kilograms (120 pounds) of ammonium bifluoride, technical
grade, to help dissolve certain iron and silica scales.
c) Add 3.79 liters (1 gallon) of wetting agent, Military Specification MIL-D-
16791, Detergents, General Purpose (Liquid, Nonionic).
d) Add the dissolved diethylthiourea, ammonium bifluoride, and wetting
agent to the diluted acid solution. Add sufficient water to obtain 3785
liters (1000 gallons).
Carbonate Deposits. Carbonate deposits dissolve rapidly in hydrochloric
acid, with evolution of free carbon dioxide. The escaping carbon dioxide tends to create
some circulation or agitation of the acid, which ensures the continual contact of fresh
acid with the scale. Once the carbonate has been dissolved from a mixed deposit, a