US1863868A - Chromium stripping bath - Google Patents

Description

Patented June 21. 1932 UNITED STATES PATENT OFFICE 4 JOHN F. MCC'ULLOUGH, OF DETROIT, MICHIGAN, ASSIGNOR 'I'O 'IERN STEDT MANUFAC- TURING COMPANY, OF DETROIT, MICHIGAN, A CORPORATION OF MICHIGAN cnaomuu srairrme BATH- have to be plated over again. It has been customary to use a caustic soda solution bath in doing this stripping, but it has been unlformly found necessary after the stripping to in some way work the nickel surface underneath, as by bufling, in order to prepare it for again receiving a deposit of chromium. In short, the nickel-plated surface is passive, and this I believe is due to the presence on the nickel. surface of an invisible oxide formed by the alkali chromate ion, Cro At any rate, the

surface after such a stripping action is substantially passive, and if it does receive any deposit of chromium this chromium is quite likely to peel oif.

- It is the object of my discovery to provide a stripping bath which will successfully remove the chromium and will not render the under surface passive to a second deposit of chromium. This I accom'plish by using a conducting acid aqueous solution which has the characteristic of reducing the chromic acid formed to a trivalent chromium. This-prevents the forming of the alkali chromate ions ness of the under. surface;

I find that substantially any of the organic acids which is a good electric conductor, and which is formed of hydrogen, carbon, and oxygen, and which is capable of reducing the CrO ,'the-h'exavalent form. of chromium, to a trivalent form, will answer the purpose. I find that oxalic acid, tartaric acid, and citric which I believe to be the cause of the passiveacid are most useful in this connection. For

instance, with oxalic acid I preferably use an aqueous solution of -120 grams per liter of water. The temperature range should be from, 77 to. degrees Fahrenheit; voltage, 12 volts in full load, using the article to be stripped as the anode. The chromium then me: serial No. 844,918.

dissolves in its hexavalent'state as chromic acid, and is then immediately reduced to the trivalent form by the oxalic acid, and thus the nickel plate is not rendered passive by the hexavalent chromium ion. The. reaction is as follows: At the anode Cr+3O:=Gr0 chromic acid; using oxalic acid, H C O we have the reaction:

201'0; (hexavalent)+6H:Cz0|-Cr|(0zO4)3-H3CO;+6H|O.

Using tartaric acid, C H O. the reaction is as follows:

20150:(hflX8V8l0nt)+24Ht0t= 2Cr+++(trivalent)+4HzO0:+400:+2H:0. Using citric acid, H C H O the reaction is as follows:

smegma-20:0, (hexava1ent)='= icr+++(trivalent)+ooo,+omo+e(cm)2co 00am. Qxalic acid is cheaper than citric or tartaric acid, and also a better conductor of electricity.

I might add that'hydrochloric acid has been used in stripping, but thisattacks' the ,base metal, such as die-cast metal, and it is also much slower than oxalic acid. 1 What I claim is: I

1. An electrolytic bath for stripping chromium plated articles by placing the article at the anode, comprising an aqueous solution of an organic electrolytic jacidic reducing agent. M

2. An electrolytic bath for stripping chromium plated articles, with the article acting as the anode of the bath, which'bath comprises an aqueous solution of an organic acid capable of reducing chromium from the hexavalent to the trivalent form, and forming a good conductor of electricity, said solution being a constituent of a class consisting of oxalic acid, tartaric acid, and citric acid.

3. An electrolytic bath for stripping chro-' mium plated articles, with the article as an anode, comprising an aqueous solution of oxalic acid. 1.

4. An electrolytic bath for stripping chr6- mium plated articles, with the article acting as an anode, comprising an aqueous solution of oxalic acid of from 60-120 grams of the acid per'liter of water, and with a tempera; ture range of from 77 degrees Fahrenheit of approximately; 12 volts.

2 1,aos,soa

to 85 degrees Fahrenheit, and with a voltage 5. An electrolytic birth for anodically stripping 'a chromium plated article, comprising i I an aqueous solution of a substantial quantity of an organic acid electrolyte that is substantially passive to the base metal of thesaid chromium plated article.

In testimony whereof I have aflixed my signature.

J OHN F. MGCULLOUGH.