Citric Acid Passivation of Stainless Steel: ASTM A967 Guide

What it does

Passivation removes free iron so the stainless can protect itself

Stainless steel resists corrosion because of a thin chromium oxide layer on its surface. Machining, welding, grinding and handling leave free iron on that surface. The free iron rusts, stains and starts pitting. Passivation dissolves the free iron and lets the chromium oxide layer form again, evenly, across the whole part.

Passivation is not cleaning. Oil, coolant and shop soil stop the acid reaching the metal, so the part has to be cleaned first. That is why a passivation line always starts with a degreasing stage.

Citric or nitric

Why citric acid replaced nitric on most new lines

	Citric acid	Nitric acid
Specifications	Recognised by ASTM A967 and AMS 2700	Recognised by ASTM A967 and AMS 2700
Handling	Not fuming, no NOx	Fuming acid, NOx risk, more PPE and extraction
Waste	Simpler neutralisation and disposal	Nitrate-bearing waste, harder to dispose of
Evidence	NASA comparative studies found citric equal to or better than nitric on the alloys tested	The long-standing reference method

The standard

ASTM A967 citric acid treatments

ASTM A967 sets out citric treatments by temperature and time. The hotter the bath, the shorter the minimum immersion.

Treatment	Citric acid	Temperature	Minimum time
Citric 1	4 to 10% by weight	60 to 71 °C	4 minutes
Citric 2	4 to 10% by weight	49 to 60 °C	10 minutes
Citric 3	4 to 10% by weight	21 to 49 °C	20 minutes
Citric 4	Other combinations of concentration, temperature and time, with or without additives, that pass the specified test
Citric 5	As Citric 4, with the bath held at pH 1.8 to 2.2

Summarised from ASTM A967. Your drawing or customer specification decides which treatment and test apply; read the current edition of the standard before you write a procedure.

The process

Citric acid passivation procedure, stage by stage

Degrease. Ultrasonic cleaning in an alkaline cleaner such as Brulin AquaVantage 815 GD takes off oil, coolant and soil. ASTM A380 covers the cleaning practice.
Rinse. Stops the alkaline cleaner carrying into the acid bath.
Passivate. Immersion in the citric bath at the treatment's temperature and time. Ultrasonics keeps fresh solution at the surface, inside bores and under weld spatter.
Rinse twice. DI or RO water clears the acid. A final rinse in pure water avoids spotting.
Dry. Hot-air drying, so the part leaves the line dry inside and out. See Rankine parts dryers.
Verify. Test against the specification and record the result.

Testing

How passivation is checked

ASTM A967 lists the verification tests. The specification for the part says which one applies.

Water immersion test
High humidity test
Salt spray test
Copper sulphate test
Potassium ferricyanide and nitric acid test, often called the ferroxyl test
Damp cloth test
Boiling water immersion test

On real parts

What a trial on welded pipe showed

Before designing a passivation line for a pipe fabrication shop, we ran trials on the customer's own stainless, mild steel and cupronickel pipe, all carrying weld marks and carbon.

Stage	Chemistry	Temperature	Time
Ultrasonic degrease	Brulin AquaVantage 815 GD at 10%	60 °C	30 minutes
Two rinses	DI water	60 °C	2 minutes each
Ultrasonic passivation	Citric passivation chemistry at 10%	60 °C	120 minutes for the stainless
Two rinses	DI water	60 °C	2 minutes each
Hot-air dry	—	110 °C	4 minutes

The stainless pipe passed the ferroxyl test. The full line that came out of those trials, with renders and the general arrangement, is on the engineered cleaning systems page. The chemistry and the operating cost are in the citric passivation case study.

Building the line

Running citric passivation as a production line

Separate tanks. Degreasing and passivation never share a bath, and each gets its own rinse, so neither contaminates the other.
Stainless tanks and heaters rated for the acid, with temperature held inside the treatment's band.
Filtration and bath control. Concentration, pH and dissolved iron are checked on a schedule, and the bath is topped up or replaced before it drifts out of spec.
Handling sized to the part. Baskets and racks for small parts, overhead carriages for spools and long assemblies.
Bunding and drainage for the acid and the rinse water. See bunding, platforms and washbays.

For a bench-top or small batch job, a single heated ultrasonic tank per stage does the work. For production, see UltraKleen Pro and SonoLine multi-stage lines, or engineered systems for a line built into a new plant.

Questions

Citric acid passivation: common questions

What concentration of citric acid is used for passivation?

ASTM A967 Citric 1, 2 and 3 all use 4 to 10% citric acid by weight. The temperature sets the minimum time: 4 minutes at 60 to 71 °C, 10 minutes at 49 to 60 °C, or 20 minutes at 21 to 49 °C.

Is citric acid passivation as good as nitric?

Both are recognised by ASTM A967 and AMS 2700. NASA comparative studies found citric equal to or better than nitric on the alloys tested, without nitric's fuming acid and NOx handling.

Do parts need cleaning before passivation?

Yes. Oil and soil stop the acid reaching the metal. Degrease first, rinse, then passivate. Ultrasonic cleaning gets into bores, threads and weld areas that a dip alone misses.

How do you test that passivation worked?

With the test your specification names. ASTM A967 lists water immersion, high humidity, salt spray, copper sulphate, potassium ferricyanide and nitric acid (ferroxyl), damp cloth and boiling water tests.

Can Misonics build a citric passivation line?

Yes. We design and supply passivation lines from single tanks to multi-stage production lines with overhead handling, including the chemistry, trials on your parts, installation and commissioning.

Start here

Passivating parts now, or planning a line?

Tell us the alloy, the part, how many and which specification it has to meet. Technical sales will come back with the process and the equipment to run it.

Plan a passivation line Ask about a trial