Abstract

Citric acid passivation offers a promising alternative to nitric acid passivation, particularly for free machining and precipitation hardened stainless steels where the latter requires the addition of sodium dichromate. While citric acid passivation is defined in industry specifications such as ASTM(1) A967 and SAE(2) AMS2700, it is done with much less specificity than nitric acid passivation. This lack of specificity allows for conditions to be applied where inadequate passivation is achieved for some alloys. In this work, the citric acid passivation method has been explored and optimized for a range of common stainless steel alloys including a common austenitic alloy, AISI(3) 304L (UNS S30403), an austenitic free-machining alloy, 303 (UNS S30300), a ferritic free-machining alloy, 416 (UNS S41600), and precipitation hardened alloy, 17-4 PH (UNS S17400). A series of commercially available chemistries have also been explored.

Introduction

Traditional solutions for the chemical passivation of stainless steel are nitric acid based, with the addition of sodium dichromate as an inhibitor for precipitation hardened and free machining stainless steels. These passivation chemistries are difficult to handle from an environmental health and safety point of view, particularly the dichromate inhibited versions. Citric acid passivation has been pursued as a replacement for both nitric acid and inhibited nitric acid based chemistries for many years, and has been incorporated into consensus specifications such as ASTM(1) A9671 and SAE(2) AMS2700.2 Adopting this process in a high-consequence production environment requires that the ability of the process to effectively passivate a contaminated surface (i.e., remove iron contamination, surface incident MnS inclusions, etc.) for a variety of stainless steels be demonstrated, particularly for free machining and precipitation hardened stainless steels. The primary driver for this work is the prevalence with which passivation of product by various vendors has been demonstrated to be deficient when using nitric acid passivation. As will be discussed below, the consensus specifications provide less guidance on appropriate combinations of solution concentration, temperature and time, raising the concern that changing the passivation methodology from nitric to citric acid could result in a greater number of performance issues due to errors when conducting citric acid passivation.

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