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To effectively passivate the free-machining stainless steels, Carpenter has developed the A-A-A (alkaline-acid-alkaline) process that neutralizes trapped acid. This method of passivation can be accomplished in less than 2 hours. Here is the step-by-step procedure:
1st Step : After degreasing, soak the parts for 30 minutes in a 5 percent solution of sodium hydroxide at 160ºF to 180ºF (71º C to 82º C).
2nd Step : Then rinse the part thoroughly in water.
3rd Step : Next, immerse the part for 30 minutes in a 20 percent-by-volume nitric acid solution containing 3 ounces per gallon (22 g/liter) of sodium dichromate at 120ºF to 140ºF (49º C to 60º C).
After removing the part from this bath, flush it with water, then immerse it in the sodium hydroxide solution for another 30 minutes. Rinse the part again with water and dry it, completing the A-A-A method.
Procedure of Passivation :
1. Isolate the exchanger and set up a circuit with an external tank with heating coil inside and a pump to circulate cleaning solution.
FLUSHING:-
2. Flush the Exchanger with fresh water to remove loose material (like dirt etc.) till such time clear water comes.
INHIBITED ACID CLEANING:-
3. Circulate inhibited Acid (5-6 % Citric Acid and 1% Rodine 213) along with non ionic wetting agent like RXSOL-T300 through Exchanger Loop for 6 to 8 hours
4. It total iron concentration reaches 5000 - 8000 PPM and concentration below 2% acid percentage then dump the cleaning solution and repeat the cycle with fresh inhibited acid.
5. Samples to be tested for pH, TDS and iron on hourly basis.
6. Normal operating temperature should be 50-60º C.
7. When iron concentration and acid concentration of solution reaches steady state and cleaning in completed. Drain the cleaning solution.
NEUTRALIZING:-
8. Circulate 1-2% solution of soda Ash for one to two for neutralization.
9. Rinse the neutralizing solution with water till pH of Inlet and Outlet is same.
PASSIVATION:-
10. Immediately fill and Circulate with 1-3% of solution of sodium Hexametaphosphate for 4-6 hours keeping operating temperature around 50-60º C. Analyze for Phosphate levels in Inlet and Outlet. (Normally maintain 30-40 PPM O-PO4 level)
11. Drain the passive solution without flushing with water.
DRYING:-
12. Dry the circuit with dry air/N2.
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