# Testing/analyzing Baths for Passivation of Stainless Steel

(2001)

Q. I searched the Internet extensively looking for current testing procedures for passivation tank chemicals and came up empty. We have a cold Nitric bath and a hot Dichromate/Nitric bath for various stainless steels. There are a few titrations that are performed on a period basis and I would like to know where these test instructions can be found. We perform tests now, but I would like to reference the latest specifications in our manual. Any suggestions are appreciated.

Thanks,

Steve Szakacs- Linwood, Pennsylvania, USA

(2001)

A. Here is a procedure for the Dichromate Determination:

SCOPE: To determine the concentration of Sodium Dichromate in a solution.

EQUIPMENT:

25 mL Buret

250 mL Erlenmeyer Flask

Deionized Water

0.1 N Sodium Thiosulfate

100 mL Graduated Cylinder

0.5% Starch Indicator

10 mL Graduated Cylinder

Conc. Sulfuric Acid

10% w/v Potassium Iodide

1.000-1.250 Hydrometer

500 mL Graduated Cylinder

PROCEDURE: Using the pipet, transfer 5 mL of the sample into the Erlenmeyer flask. Using the graduated cylinder, add 100 mL deionized water to the flask. Using the graduated cylinder, add 5 mL sulfuric acid and 10 mL potassium iodide solution to the flask. Titrate with 0.1 N sodium thiosulfate until the solutions turns to a straw color. Add 2 mL starch indicator to the flask and continue to titrate until the blue-black color disappears. Record the mL used as V1. Place 500 mL of sample in the graduated cylinder. Measure the density of the solution with the hydrometer. Record this value as "P".

CALCULATION:

C1 = Concentration of Sodium Dichromate in %w/w.

V1 = mL of sodium thiosulfate used in the titration

F1 = 2.483

N1 = Normality of sodium thiosulfate used in the titration.

P = Density of the solution

REPLENISHMENT:

W2 = (C1/100)*W1

W3 = (C2/100)*W1

W4 = W3-W2

V3 = W4/453.6

V2 = Volume of the solution in milliliters

W1 = Weight of the solution in grams

C2 = Optimum concentration of Sodium Dichromate %w/w.

C1 = Actual Concentration of Sodium Dichromate in %w/w.

W2 = Weight of Sodium Dichromate in solution in grams.

W3 = Optimum weight of Sodium Dichromate in grams.

W4 = Weight of Sodium Dichromate to add in grams

V3 = Weight of Sodium Dichromate to add in pounds.

Kansas City, Missouri

(2001)

A. There are several books that can be ordered through ASM International and AESF that provide procedures for analysis of metal finishing solutions. Here is a procedure for the determination of Nitric Acid:

SCOPE: To determine the concentration of Nitric acid in a solution. EQUIPMENT:
Ira Donovan, M.S.F.

Kansas City, Missouri

October 15, 2010

Q. I have a passivation line that is 20-25% Nitric acid at 120 °F. Is there a simple tester that I can use to monitor concentration? I would prefer something you could just dip in the tank and measure the concentration. A glass hydrometer would work, but the operator would have to handle the acid.

Thanks,

Joe

- Hatfield, Pennsylvania USA

March 9, 2012

A. Working through the math of the nitric acid procedure, V1 mL of N1 NaOH can neutralize V1*N1 mmol of HNO3, which is equal to V1*N1*(63.01) mg of HNO3.

Dividing by the number of mL in the sample produces a concentration (for a mL sample) of V1*N1*12.6 mg/mL, which produces a value of V1*N1*1.26 %w/v, not %v/v.

The Canning Handbook (pg. 772 of the 21st edition) supports this correction, and provides correct factors to find %v/v terms of concentrated solutions of nitric acid (1.46 for SG 1.38 and 1.30 for SG 1.41).

Joshua MeisnerAerospace - Long Island, New York

November 20, 2014

Q. I see in the Equation above for sodium dichromate concentration that we use a factor F1. Where does that factor come from? I'm curious to know how we determine that factor.

Thank you

Peter

metal finishing - paterson New Jersey USA

December 1, 2014

A. Peter,

The F1 constant in the titration equations is just hiding all of the other math involved. If you run the numbers from scratch and trace through all the stoichiometry, dilutions, etc., the number given for F1 should appear. In procedures such as this, the math is typically "precooked" since the intended user is only interested in the final answer and might not have been a chem major.

December 3, 2014

Q. Is there a book or something else that will step through the math to get this F1 number? I have seen this number (2.483) in multiple places, but I need to see how this number was calculated.

Thanks,

Julie

- Tucson, Arizona, USA

December 12, 2016

Q. Is there someone that can explain to me how to get the F1 constant? I have been trying to find it with all the dilution, titration calculations but cannot get the F1 constant. Please help! Thanks

Nathania Yuliani- Santa Ana, California

December 15, 2016

I have to confess, I don't know where 2.483 comes from either. I'm getting a totally different number.

I managed to find something that explains the reactions involved at

http://www.meduniv.lviv.ua/files/kafedry/tokshim/English/Analytical/Manuals_Analytchem/Iodometry.pdf

Essentially, adding the Potassium Iodide and Sulfuric Acid generates 3 moles of iodine (I2) for every mole of dichromate in solution. The iodine is then titrated by the Thiosulfate with an equivalence of 2 moles Thiosulfate per mole of iodine.

The normality of the thiosulfate solution would seem to be equal to molarity (mol/L) here.

So you have your mL of titrant (V1), divide by 1000 to convert it to liters, multiply by the molarity (N1, though assigned as 0.1 in the procedure) to convert to moles of thiosulfate titrant, divide by 6 to convert moles thiosulfate to moles dichromate, and multiply by the molar mass of sodium dichromate (261.97 g/mol) to get the grams of sodium dichromate present in the original 5 mL sample. Now for %w/w, you need to divide that by the total mass of that original sample, which is its density times 5 (for the 5 mL).

Stringing all that together, the F1 in %w/w = V1*F1*N1/density

would presumably be 261.97/(1000*6*5) = 0.0087323

I rather suspect I'm missing something here.

Stellar Solutions, Inc.

McHenry, Illinois

December 15, 2016

I have tried carbon treating my cyanide bath. It has helped a bit. But the problem is not solved completely. At low current density areas. There is pitting like patterns. The problem is coming on flat surfaces at low current density areas.

At high current density areas, there is no pitting.

The pitting looks like small dots all over the surfaces of low current density areas.

I am not using Rochelle salt at all.

My bath is completely based on copper cyanide and sodium cyanide.

There was no such problem in the beginning.

Whenever problem arises I add 5kgs of copper cyanide and 7.5 kgs of sodium cyanide. It generally resolved my issues in the past.

But now carbon treating and additions of copper cyanide and sodium cyanide iss not helping.

While carbon treating I had used 500grams to 650 litres of solution.

It is adamant to the solution?

I had also tried dummying.

In the dummy also small bubbles like roughness is appearing on low current density areas.

Is this pitting is because of impurity or I need to add some copper cyanide and sodium cyanide to the solution.

Please help me out.

- jalandhar, punjab, india

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