Methods for passivating stainless steel

Abstract

Methods for passivating stainless steel after acid pickling treatment in the absence of nitric acid are provided. The methods include the steps of cleaning the pickled stainless steel with an alkaline composition to obtain clean steel, activating the clean steel with an activator composition to obtain activated steel, and passivating the activated steel with a passivating composition in the absence of nitric acid. In the preferred embodiment, the activator composition contains at least one activator, the activator having a significantly higher binding affinity for iron than for chromium as evidenced by their metal complex formation constants.

Description

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 60 / 426,147 filed Nov. 14, 2002, the disclosure of which is expressly incorporated herein by reference.[0002] The present invention relates generally to methods for passivating stainless steel, and more specifically to pickling and passivation of low chromium steel in the absence of nitric acid.BACKGROUND AND SUMMARY[0003] The industry standard for pickling stainless steel is to use a hot solution of nitric acid and hydrofluoric acid. Such pickling processes produce hydrofluoric acid vapors and waste nitrates that require special disposal procedures. Alternatives to such processes have been explored, including mechanical abrasion and electrolytic pickling methods. One alternative, disclosed in U.S. Pat. No. 5,821,212, assigned to Crown Technology, Inc., to Peterson (Rinse Aid and Process for Stainless Steel) involves a peroxide-based rinse additive for cleaning stainless steel following pickling wit...

AI Technical Summary

Benefits of technology

0038] To determine the efficacy of the passivation bath (Bath 3) during periods of concentration fluctuations, a series of baths were prepared to simulate lean and rich conditions and evaluated versus the passivation treatment described previously. The results are shown in Table 3 below.

0039] It is important to note that the importance of the iron to the passivation process is to act as a redox buffer. The hydrogen peroxide addition oxidizes ferrous iron to ferric iron, which helps maintain the redox potential of the passivation bath at 500 to 600 mV vs. the Ag / AgCl reference electrode. As more stainless steel is processed, ferrous iron builds in the passivator bath and lowers the redox potential. The redox potential of the bath can be easily monitored using an ORP probe that is set to trigger the addition of small volumes of hydrogen peroxide that oxidizes the ferrous iron to ferric. In this way the redox potential of the passivation bath can be maintained in the optimal range.

0040] An activator composition containing the following was prepared: 25 g / L 85% w / w phosphoric acid; 10 g / L sodium fluoride at pH=2.56. A second activator solution containing the following was prepared: 21.8 g / L oxalic acid dihydrate; 10 g / L sodium fluoride with 50% w / w caustic soda added dropwise to a pH of about 10-11. Activator baths were prepared, which contained the activator compositions of this Example.

0041] Stainless steel coupons were treated using the activator baths of this Example with Baths 1 and 3 as described in Example 2. Salt spray tests showed excellent results.

Problems solved by technology

These options can be hazardous and lead to significant waste treatment costs.

However, citric acid passivation has not achieved the desired results.

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