Wednesday, November 12, 2014

General Instruction for Chemical Cleaning




1.        Introduction
The purposes of this specification are to remove impurities
on the metal surface an well as to allow the rust preventive effects to be fully displayed so that the quality of products manufactured at industrial machinery plants is assured, and the specification prescribes treatment in general when chemical cleaning is carried out as a means to remove rust.
This document gives precautions on general acid cleaning procedures and for the type of solvents, concentration, rate of dilution, time, and other details, standard specifications or each acid cleaning specialists shall be referred.

2.          General Precautions

(1)      Chemical cleaning shall be carried out after all the work processes (including hydraulic and air tight tests) are completed and after chemical cleaning, any processing, in particular, accompanied by heating shall not be carried out.
If hydraulic and air tight tests are needed to be carried out after chemical cleaning for convenience of manufacturing, rust preventive means such as application of rust preventive additives shall be provided to prevent rust formation.
(2)    Before chemical cleaning, spatters by welding, slag, chips at the time of machining, dust, and other impurities shall be thoroughly removed.
(3)    Products shall be handled carefully, and utmost care must be taken to prevent flaws and damage to products.
(4)    When chemical cleaning is carried out, treatment liquids and immersion time shall accurately be controlled and utmost care shall be taken to prevent accidents by over pickling.
(5)    If immersion cleaning is carried out in the chemical cleaning process, air bleeding inside products shall be taken into account and the immersion direction and method shall be determined in such a manner so as to prevent generation of air pots.
(6)     The metal surface after chemical cleaning shall be free from any residual acid liquid and shall exhibit pH7.
(7)    Products finished with chemical cleaning shall not be stored near acid cleaning tanks or in a dusty location, and shall undergo rust-preventive treatment promptly after drying.

3.            Operation Procedure and precautions
The acid liquid operation process slightly differs depending on materials but, in principle, comprises the following procedures:
(1)    Degreasing
(2)    Water rinsing
(3)    Acid cleaning
(4)    Water rinsing
(5)    Neutralization
(6)    Water rinsing
(7)    Drying

3-1   Degreasing
When grease, rust, preventive oil, cutting oil, machine oil, or oily substance such as grease or oil degradation products are coated or adheres to the metal surface, the oily substance shall completely be removed by wiping or immersion
*          Reference
Chemicals:     sodium hydroxide, (caustic soda), sodium hydrogen carbonate, sodium tripolyphosphate, sodium metasilicate, surfactant, etc.
Treatment temperature     :          50-60
Treatment time                     :        3-5 hours




3-2    Water rinsing
(1)        Water rinsing shall be carried out only when degreasing is performed.
(2)        Using a water hose, spray-brushing or immersion-brushing is carried out to completely remove degreasing liquid. It is effective to spray pressurized water to the work piece treated with liquid.
(3)    When immersion rinsing is carried out, allow water to constantly flow to prevent grease compounds or treatment liquid from floating up on the water surface.
(4)    Wetting on the metal surface of the work just after water rinsing shall be expansion wetting.(If breakage of water coverage or water repelling portion is visually recognized1 re-treatment is required for insufficient degreasing.)

3-3    Rust removal
(1)    In principle, the work piece shall be immersed in a tank filled with treatment liquid. However, the excessively large work piece may be treated by filling, brushing, or coating.
(2)    The inhibitor shall be added to the acid liquid, for a uniform beautiful finish surface, free from smut, and reduction of metal loss.
 *  Reference
Hydrochloric acid concentrations of 5 to 15% are, in general, used at room temperature.
                                 The treatment time is 2-5 hours.



3-4    Water rinsing
(1)    Using a water hose, water-rushing or immersion-rushing is thoroughly performed to remove the acid liquid.  It is an effective method to spray pressurized water to the work piece.
(2)    Water must be allowed to constantly flow during immersion-rinsing -

3-5    Neutralizing
(1)    The neutralizing liquid which neutralizes and simultaneously has a temporary rust preventive effect (ability to prevent rust formation for about 3-7 days if water is not splashed) shall be used. However, it shall not have any detrimental effects on adhesion of coating when the metal surface Is coated after the neutralizing liquid is dried.
(2)    The work piece shall, in principle, be immersed in a tank filled with the neutralizing liquid. However, excessively large work pieces may be neutralized by filling or brushing but care must be taken to allow thorough neutralization to take place.
(3)    Heating the neutralizing liquid is an effective means to promote neutralization and accelerate drying.

3-6   Drying
(1)    Drying after chemical cleaning is carried out to enable perfect and effective subsequent surface treatment. The work piece must be dried immediately after cleaning.
Heating the neutralizing liquid is an effective method to achieve quick drying.
(2)    The cleaned work piece shall be dried in a clean location, free from dust, to prevent adhesion of impurities to the clean metal surface.

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