ASTM B912, “Standard Specification for Passivation of Stainless Steels Using Electropolishing” is the next topic in our blog series on specifications. This is a widely accepted specification on how to properly establish an electropolishing process for stainless steel to generate a passive surface. Requests to process according to ASTM B912 are received by Astro Pak from a wide range of industries. The standard was originally published in 2002 and, as of this post, was most recently updated in 2018.
Why Passivate Post Electropolishing
Two major points must be made about ASTM B912 before a detailed explanation of the standard begins. First, it is true that a sound electropolishing operation results in a passive stainless steel surface. However, Astro Pak’s technology team has found that a passivation treatment following electropolishing will provide an even greater benefit to the stainless steel’s corrosion resistance. The graph below details the increased benefit in the form of the passive layer’s chromium-to-iron ratio.
Second, ASTM B912 is not a procedure. The standard is not meant to provide process parameters such as: electrolyte solution chemistry, electrical contact time, or the required current density to adequately polish a stainless steel surface. The goal of the standard is to provide a framework for an electropolishing provider to qualify a self-developed electrochemical process via cleaning recommendations, acceptance criteria, and verification tests.
ASTM B912 Specifics
ASTM B912, like other ASTM standards, begins with sections on the specific scope, referenced documents, and terminology relevant to the standard. Ordering information is the first section of non-standard ASTM Standards content. In this section, the standard specifies it is the responsibility of the purchaser to determine the acceptable electropolishing process for the following:
- finish (bright vs. dull)
- level of contact marks
- minimum and maximum amount of metal removal
Due to the language in this section, there is no universal requirement per ASTM B912 for a minimum amount of metal removed nor a minimum surface finish in terms of reflectivity or roughness for the electropolishing operation to be considered complete. The only requirement set in the standard for the electropolishing procedure is that the chosen parameters result in a component that passes one of the verification tests mentioned later in this post.
Section 5 of ASTM B912
Section 5 of ASTM B912 introduces the actual electropolishing operation. Before any electrochemical work, the standard specifies to preclean the components per ASTM A380 or ASTM B322, “Guide for Cleaning Metals Prior to Electroplating.” No process parameters are mentioned in Section 5.5.2, “Electropolishing,” the section explains that parts should be electropolished at the current, temperature, and time necessary to provide both a passive surface and previously agreed upon surface finish. Although ASTM B912 is not meant to be a procedure, it does contain a basic set of process parameters suitable for multiple stainless steel alloys in its Appendix.
The electrolyte solution used in electropolishing often leaves a residual phosphate film on the workpiece that must be removed. ASTM B912 recommends rinsing the piece in a nitric acid solution to remove this film as a “Post Dip” operation. Nitric acid is not mandated, other acids or chemical solutions that maintain a passivated surface are allowed according to Section 184.108.40.206 of the standard. The final operation mentioned in Section five is to rinse the polished workpiece. ASTM B912 does not provide a rinse procedure but recommends that deionized or distilled water are used to remove all acidified water left on the surface during the electropolishing and post dip operations.
Following the rinse procedure are passivation verification tests. All five of these tests are the same as the first five introduced in our previous blog post on ASTM A967. Like mentioned in the previous post, the verification tests are qualitative; they do not show how corrosion resistant the passive layer is. Instead, they determine whether a passive surface is present.
Five more sections of content round out ASTM B912. They are:
- Part Sampling
- Part Rejection
- Process Test Report
The first three are intended for electropolishing operations in which components are batch processed, addressing how quality can be maintained at scale when inspecting every processed component is not feasible. The test report is documentation certifying that components have been tested in accordance to one of the verification tests mentioned above. It is not a comprehensive report on the full electropolishing procedure performed.
ASTM B912 and Astro Pak
Proper following of ASTM B912 results in a repeatable and verifiable electropolishing process for stainless steel. Astro Pak can utilize our processes developed from this standard in all of our shop facilities across the nation, or via our electropolishing crews that mobilize to your location when systems are too large to transport or cannot be easily disassembled. Astro Pak also provides a robust documentation package upon completion of all jobs to serve as detailed evidence of the work performed on your system. Contact Astro Pak today for more information about our electropolishing capabilities.