---
title: "Passivation Spec: Stainless Steel per ASTM A967 | OpenSpindle"
description: "Stainless steel passivation per ASTM A967: how nitric and citric acid remove free iron and restore the chromium oxide layer, plus when it is required."
canonical: https://openspindle.com/reference/passivation-spec
---

# Passivation Spec Guide

What stainless steel passivation does and does not do, nitric vs citric acid methods per ASTM A967, when it is required, and how the result is tested.

Passivation is a chemical treatment that cleans a stainless steel surface and restores its natural corrosion resistance. It removes free iron and other surface contaminants left by machining, forming, or handling, which lets the chromium in the alloy re-form a thin, self-healing chromium oxide layer. Passivation is not a coating: it adds no measurable thickness and no color, and it does not change dimensions. The governing standard in North America is ASTM A967, which defines nitric acid, citric acid, and electrochemical methods.

## What passivation does and does not do

| Passivation does | Passivation does not |
| --- | --- |
| Remove free iron and surface contamination | Add any coating or measurable thickness |
| Restore the chromium oxide (passive) layer | Change part dimensions or tolerances |
| Improve corrosion resistance of stainless | Add color or a decorative finish |
| Clean embedded iron from tooling and handling | Fix pitting, deep corrosion, or heavy scale |

*Passivation is a cleaning and conversion step, not a plating or paint. Heat-tint and scale from welding usually need pickling first.*

## Nitric vs citric acid methods (ASTM A967)

ASTM A967 allows immersion in nitric acid, immersion in citric acid, or electrochemical treatment. Nitric is the traditional method; citric is a lower-toxicity, easier-to-handle alternative that has become widely accepted and gives equivalent results on most alloys.

| Method | How it works | Notes |
| --- | --- | --- |
| Nitric acid | Nitric bath, sometimes with sodium dichromate; several nitric types by concentration and temperature | Traditional; strong oxidizer; more hazardous handling and waste |
| Citric acid | 4-10 percent citric solution; Citric 1-5 vary by time and temperature | Lower toxicity, safer disposal, equivalent results on most grades |
| Electrochemical | Applies current in an electrolyte to accelerate passivation | Used for specific parts and higher throughput |

## When passivation is required

Passivation is called out whenever machining, grinding, or handling can embed iron in a stainless surface, or when the alloy's corrosion resistance is critical. It is standard on medical devices and implants, food and pharmaceutical equipment, aerospace and defense hardware, and any stainless part that will see a corrosive or wet environment. Free-machining grades and lower-chromium martensitic stainless can be harder to passivate and may need adjusted methods.

## How passivation is verified

ASTM A967 does not verify passivation by measuring a layer, since it adds none. Instead it specifies corrosion and cleanliness tests to confirm free iron was removed. Common acceptance tests include the water immersion test, high humidity test, salt spray, copper sulfate test, and the potassium ferricyanide-nitric acid test. The drawing calls out which test and duration apply.

Details reflect ASTM A967 / A967M and common practice. The equivalent legacy spec QQ-P-35 is now inactive and often mapped to A967. Confirm the method (nitric type or citric process) and acceptance test with your finisher before release.

## On OpenSpindle

- [Stainless steel machining](https://openspindle.com/materials/stainless-steel.md)
- [Anodizing spec chart](https://openspindle.com/reference/anodizing-spec.md)
- [Surface finishes compared](https://openspindle.com/reference/surface-finishes-compared.md)

## Frequently asked questions

### Does passivation add thickness or coating to a part?

No. Passivation is a chemical cleaning and conversion treatment, not a coating. It removes free iron and lets the chromium oxide layer re-form, but it adds no measurable thickness, no color, and does not change dimensions or tolerances.

### What is the difference between nitric and citric acid passivation?

Both are allowed by ASTM A967 and restore the passive layer. Nitric acid is the traditional, strongly oxidizing method with more hazardous handling and waste. Citric acid is a lower-toxicity, easier-to-dispose alternative that gives equivalent results on most stainless grades, which is why many shops have switched to it.

### When is stainless steel passivation required?

Whenever machining or handling may embed iron in the surface, or when corrosion resistance is critical. It is standard on medical and implant components, food and pharmaceutical equipment, aerospace and defense parts, and any stainless part exposed to a corrosive or wet environment.

### How is passivation tested?

Because it adds no layer to measure, ASTM A967 verifies passivation with corrosion and cleanliness tests that confirm free iron is gone. Common ones are water immersion, high humidity, salt spray, copper sulfate, and the potassium ferricyanide-nitric acid test. The drawing specifies which test and duration apply.