? Where is your part right now?

Not made yetOne shop: we machine the part and passivate it in the same order, so the free iron left by machining is dealt with before the part is packed. Nothing is added to the surface and no dimension changes.See the processes we run →Already madeSend the finished parts. Passivation is a dip and a rinse, so it reaches internal surfaces as well as outer ones - which is exactly where machining contamination is hardest to see.Check if my parts qualify →Still comparing finishesPassivation does not build a layer - it removes what should not be there and lets stainless steel protect itself. If you need thickness, colour or hardness, it is the wrong process.Which process do I need? →

SURFACE FINISHING

Passivation Service

Stainless steel only No size change Matte, colourless Removes free iron

A chemical clean for stainless steel that removes free iron and surface contamination left by machining, so the metal can rebuild its own chromium oxide layer. Our published scope for this process is stainless steel, the published texture is matte, and the treatment is a dip - so internal surfaces are reached as well as outer ones. Nothing is added and nothing is coloured.

Get a quote Which steels are treated?

Stainless steelThe published material for this process

Matte textureAs published for this process

No size changeA chemical clean, not a coating

One published materialStainless steel in the specification table

A batch of stainless steel tubular machined parts standing upright after passivation, showing a clean matte surface — Passivation does not change how a stainless part looks much - it changes what is on the surface. The treatment matters most on the faces you cannot see.

Where it fits

Passivation comes after machining - because machining is what puts the free iron there

Stainless steel protects itself with a chromium oxide layer. Machining, forming, blasting and handling all leave free iron and foreign particles on the surface, and those are what rust first or seed corrosion. Passivation removes them.

STEP 1Machine and de-burrCutting tools smear free iron into the surface - the reason the step exists

STEP 2CleanOil, shop soil and polishing compound all have to come off first

STEP 3Remove the free ironThe published description calls this a dip in an acid bath

STEP 4Rinse and let the layer reformYou are here - the chromium oxide layer rebuilds in air once the surface is chemically clean

STEP 5Inspect and packInspection looks for stains, water spots and any remaining contamination

The order matters because passivation is a clean, not a coat. Anything that re-contaminates the surface after the bath - a carbon steel brush, a dirty bench, an unprotected storage rack - undoes it. That is why handling after treatment is part of the specification. See drawing notes.

A heap of stainless steel turned pins and studs with a clean bright finish after passivation

Passivation is the last operation on stainless parts like these - the parts come out chemically clean and are packed without being touched by bare hands.

A tall container filled with passivated steel rivets, showing the clean grey surface across the batch — Volume parts are passivated in bulk. The treatment is a dip, so a container full of rivets is treated in one pass rather than part by part.

Two stainless steel strips with visible weld zones, showing the heat tint that pickling removes before passivation — A weld leaves a heat tint and free iron. Pickling removes the scale; passivation then deals with what is left in the surface - the two are different steps.

Fit check

Can we passivate your part? Three things decide it

The grade, what happened to the surface before treatment, and how the part is handled afterwards. The third one is what most often undoes a perfectly good passivation.

Grade

Published scope - stainless steel is the one material named in our table for this process
Different stainless families (austenitic, ferritic, martensitic, duplex) respond differently, and the acid system has to suit the family
Free-machining grades contain sulphur, which changes the surface after machining
Tell us the grade, not just "stainless"

Send the material spec with the drawing and we will confirm the grade against this process.

What the surface has been through

Machining and welding leave free iron - that is what passivation is for
Blasting, brushing and polishing media can embed a foreign metal in the surface
A weld heat tint usually needs pickling first, before passivation
The acid system for the bath is being confirmed per grade rather than assumed

Tell us what operations the parts have already been through.

Handling after treatment

A passivated surface is chemically clean - it must not be touched or wiped with anything that re-contaminates it
A carbon steel brush, a dirty bench or a mild steel rack will undo the treatment
Parts are packed for shipping straight after inspection
Say if the parts will be stored or handled in-house for a period first

Tell us how the parts will be handled and stored, and we will pack accordingly.

Specifications

Passivation: 1 material published, a matte finish and no size change

Every value below is what we publish for this process. Where the shop has not confirmed a figure, the line says To confirm rather than a guess.

Material (published table)

1 listedStainless steel - the only material in our specification table for this process

Colour

n/aAs published - the treatment does not colour the part

Texture

MatteAs published for this process

Effect on size

No size changeA chemical clean - nothing is added and nothing is removed dimensionally

Standards

To confirmThe published table has an empty Standards row; the standard this process is run to is being confirmed

Acid system (nitric / citric)

To confirmThe published description calls it an acid bath; which acid system is used per grade is being confirmed

Result on the surface

Published claimThe description states it enhances appearance and functionality and keeps the material from rusting for a very long time

Effect on later processes

Published claimThe description states that painting and polishing are made easier and more effective afterwards

The line that matters most: passivation is a clean, not a coat. It cannot repair a scratch, cannot hide a weld tint and cannot make a part that has already rusted look new. What it does is remove the contamination that would have started corrosion - which is why it is specified after machining and welding, not instead of them.

What it changes

Nothing is added and nothing is removed dimensionally - no allowance is needed

Unlike plating, coating or cladding, this process does not build a layer and does not cut one back. That is why it is specified on parts that are already to size, including parts with threads and precision fits.

Where it acts	What changes	Why it matters	What to do
Free surfaces	Surface chemistry, not size	The published process does not change dimensions.	Nothing to allow for.
Threads	Nothing dimensional	No layer is deposited, so a thread still takes its nut.	No masking needed for fit.
Press fits and bearing seats	Still fit	The treatment is a clean and a rinse, not a build-up.	No allowance needed.
Internal bores and crevices	Reached, then must be drained and dried	A dip reaches internal surfaces - which is useful, but trapped liquid is a quality risk.	Tell us about blind features so the drainage is planned.
Welded zones	Heat tint is not removed by passivation alone	A weld leaves a scale and a tint that need pickling before the surface can be passivated.	Specify pickling before passivation on welded parts.
Appearance	Matte, colourless - no visible change	This is not a cosmetic process: it will not polish a surface or change its colour.	Decide the cosmetic finish separately - see the finish comparison.

The published process is dimensional-neutral. What it does not do is fix a surface that was already damaged, stained or welded without pickling first.

Tell us which features are already to final size - on this process they normally stay that way.

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Which process

Passivation, pickling or electropolishing - three different jobs

These three are constantly confused. Our published table covers passivation on stainless steel; the other two are separate processes with separate purposes, and the table below says which one a given problem belongs to.

Passivation

Removes free iron

The surface is chemically cleaned so the free iron left by machining and handling is removed, and the metal can rebuild its own chromium oxide layer. Nothing is added, nothing is coloured and no dimension changes - which is why it can be specified on a finished part.

Best for: machined stainless parts, parts that will see moisture or washdown, parts going into clean service

Not for: removing a weld heat tint or an oxide scale - that is pickling, which has to come first.

Two stainless steel strips with visible weld heat tint beside the cleaned metal surface

Pickling

Removes scale and tint

Pickling removes the oxide scale and the heat tint left by welding and hot forming. It is a more aggressive chemical step than passivation, and on a welded part it normally comes before it rather than instead of it.

Best for: welded fabrications, heat-tinted zones, parts with mill scale

Not for: a finished precision surface - pickling removes material from the surface and changes the finish.

A group of bright polished stainless steel machined components with a reflective finish

Electropolishing

Removes metal, brightens

Electropolishing is an electrochemical process that removes a thin layer of metal and leaves a bright, smoothed surface with the surface peaks levelled. It is a different process with a different result, and it does change size, so it is not interchangeable with passivation.

Best for: parts needing a bright surface, burr-critical parts, hygienic surfaces

Not for: a part that is already to size with a matte finish requirement - it removes metal and brightens the surface.

Tell us whether the problem is contamination, scale or surface brightness, and we will confirm which of the three the part actually needs.

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Pick or avoid

When passivation is the right call - and when it is not

Pick passivation when

The part is stainless steel and has been machined, formed or handled since it was last clean.
Corrosion has been seen on stainless parts - usually rust stains at a machined edge or a weld.
The part has to go into moist, washed-down or clean-service conditions.
Dimensions are already final and nothing may be added or removed.
The specification or the customer calls for a passivation step after machining.

Avoid passivation when

You need thickness, colour or hardness - nothing is deposited here. See electroplating or anodizing.
The part has a weld heat tint or mill scale - that needs pickling first, not passivation.
The part is carbon steel or aluminium - this is a stainless steel process. See black oxide or chromate.
The surface is already damaged or stained - a clean cannot undo a scratch or a stain.
A bright cosmetic finish is wanted - that is electropolishing or polishing, not passivation.

Not sure whether the part needs a clean, a pickle or a brightening step? Send the drawing and say what the surface has to look like when it comes back.

Get a quote

Design constraints

What decides whether the passivated part comes back right

Four variables: the grade, what happened before treatment, drainage of the part, and how it is handled afterwards.

Grade and family

Austenitic, ferritic, martensitic and duplex stainless grades do not respond to the same acid system, and free-machining grades carry sulphur in the surface. Name the grade on the drawing rather than writing "stainless".

What happened before treatment

Machining smears free iron into the surface, welding leaves scale and tint, and blasting or brushing can embed a foreign metal. Passivation addresses free iron; the others have to be dealt with before it, not by it.

Drainage of the part

Passivation is a dip and a rinse. A blind bore, a crevice or a lapped joint traps liquid, and trapped liquid is a quality risk long before it is a visible defect. Tell us about blind features.

Handling after the bath

A passivated surface is chemically clean. A carbon steel brush, a bare hand or a mild steel storage rack can re-contaminate it and undo the treatment - which is why the handling and packing are part of the specification.

Welded parts

A weld zone has a different structure and a heat tint. On welded fabrications, plan pickling and passivation as two steps, in that order, and say so on the drawing.

Cosmetic expectations

The published texture is matte and the colour does not change. If the part has to look brighter or more uniform than machining left it, that is a separate finishing operation.

Drawing notes

What to write on the drawing so the treatment is what you meant

Two lines carry the requirement: the grade, and what may not happen to the surface afterwards. Passivation is undone more often by handling than by the bath.

1.  PASSIVATE PER THIS SPEC.  FREE IRON SHALL BE    REMOVED FROM ALL MACHINED AND WELDED SURFACES.    GRADE: ____________  (NAME THE STAINLESS FAMILY)2.  WELDED ZONES: PICKLE BEFORE PASSIVATION.    HEAT TINT SHALL BE REMOVED.   (DELETE IF NOT WELDED)3.  DO NOT RE-CONTAMINATE AFTER TREATMENT:    - NO CARBON STEEL BRUSHES OR TOOLS    - NO BARE-HAND HANDLING OF TREATED SURFACES    - NO MILD STEEL RACKS OR STORAGE BINS4.  DIMENSIONS ARE FINAL BEFORE TREATMENT -    PASSIVATION SHALL NOT CHANGE SIZE.5.  DRAIN AND DRY ALL BLIND FEATURES:    ______________________  (LIST BORES AND CREVICES)6.  INSPECT: NO RUST STAINS, WATER SPOTS OR    REMAINING HEAT TINT.  PACK DRY AND CLEAN.

Examples only - replace every blank with your own requirement. The re-contamination line is the one most often left off, and it is the one that decides whether the treatment survives to the customer.

Which steels

What we passivate - one published material, and the metals that belong elsewhere

Our table for this process names one material. The rows below set out what that covers and which process the other materials belong to.

Material	Passivation	Notes	Use instead
Stainless steel	Yes	The one material in our published specification table for this process. The grade and family still decide the acid system.	—
Stainless - welded fabrications	With pickling first	Heat tint and scale have to be pickled before the surface can be passivated.	All finishes
Carbon and alloy steel	Not this process	These steels have no chromium oxide layer to rebuild; they are protected by a coating or a conversion layer.	Black oxide
Aluminium	Not this process	Aluminium is protected by anodizing or by a chromate conversion layer.	Anodizing
Zinc-plated parts	Different treatment	A plated part is protected by its plating and by the passivation of that plating - a different chemistry.	Zinc plating
Parts needing a bright surface	Different process	Passivation leaves the published matte texture and does not brighten the part.	All finishes

Send the drawing with the grade on it - if passivation is not the right step for the material or the surface condition, we will say which one is.

Get a quote

Applications

Where passivation matters most

Passivation is specified where a stainless part has to stay clean and corrosion-free in service, and where the surface that matters is often one nobody can see.

A large stainless steel vessel and pipework in a food processing plant, polished to a hygienic finish

Food, beverage and pharmaceutical equipment

Washdown, acids and cleaning agents all attack a surface that still carries free iron. Machined and welded stainless equipment is passivated so the surface it shows is the surface that resists.

A stainless steel medical trolley with shelves in a clinical setting

Medical and laboratory equipment

Repeated sterilisation and cleaning is the service condition for medical stainless. Passivation removes the machining contamination that would otherwise show up as staining or corrosion.

A group of machined 303 stainless steel pins and small turned components

Machined parts that must ship clean

Every machining operation leaves free iron in the surface. On small turned parts - and especially on free-machining grades - passivation is what makes the difference between a part that ships clean and one that shows rust spots in storage.

QA and documentation

What you get back with the parts

Purchasing usually needs paper as well as parts. Here is what we can state today and what is still being confirmed.

Available

Visual inspection

Parts are inspected for rust stains, water spots, remaining heat tint and handling marks after the bath.

Available

Clean packing

Treated parts are packed dry and clean, because re-contamination after the bath undoes the treatment.

To confirm

Standard and test method

The published table has an empty Standards row; the standard this process is run to, and any water-immersion or copper-sulphate test, is being confirmed.

To confirm

Acid system per grade

The published description calls it an acid bath. Which acid system is used for which stainless family is being confirmed.

Passivation, corrosion and stainless steel in detail

Longer answers on the questions this page only has room to state.

Passivation of stainless steel

The background on the process, the acid systems and why free iron is the thing being removed.

Passivation as a surface treatment

Where passivation sits against the other surface treatments, and what it does not do.

Corrosion resistance metals

Which metals resist corrosion and by what mechanism - useful when passivation is being specified for the wrong reason.

Corrosion vs oxidation vs rust

The three terms used interchangeably in practice, and what each one really means on a part.

Types of surface finishes for CNC machining

How a chemical clean compares with plating, coating and anodizing on a machined part.

Mastering surface treatments in machining

How the treatments are sequenced around machining, and where each one belongs.

FAQ

Passivation FAQ: the questions engineers ask us

What does passivation do?

It chemically cleans a stainless steel surface so the free iron and contamination left by machining, forming and handling are removed. Once the surface is clean, the metal rebuilds its own chromium oxide layer. Our published texture for the process is matte and the colour does not change.

Does passivation change the dimensions?

No. Nothing is deposited and nothing is cut back, so the published process does not change size - which is why it can be specified on a part that is already finished. See what it changes.

Which materials can be passivated?

Stainless steel is the one material in our published table for this process. Carbon steel, aluminium and plated parts belong to other processes - see the material table.

Will it fix a welded part?

Not on its own. A weld leaves a heat tint and an oxide scale that need pickling first. On welded fabrications, pickling and passivation are two steps in that order. See the process comparison.

Does passivation make the part shiny?

No. The published texture is matte and the treatment does not brighten a surface. If the part has to look brighter, that is a polishing or an electropolishing step, which removes metal rather than cleaning it.

Can it remove a scratch or a stain?

No. Passivation is a clean, not a repair. A scratch, a stain or an already-corroded patch stays as it is - what the treatment prevents is the contamination that would have started new corrosion.

What standard is it run to?

The published table has an empty Standards row, so we do not state one here. Which standard the process is run to, and which test method goes with it, is being confirmed with the shop - ask if the part has to be certified.

How is it different from zinc or nickel plating?

The plating processes deposit a metal layer on the part and change its size; passivation adds nothing at all. They are chosen for different reasons - see zinc plating for the plating side.

How do I get a quote?

Send the drawing with the grade on it, say what operations the parts have already been through, and name any welded zones. Contact the team with the part details and we will come back with a quotation.

Get your passivation quote

Upload a STEP, IGES or PDF drawing with the stainless grade on it. Tell us what the parts have already been through - machining, welding or blasting - and we will come back with the treatment and the sequence.

Get a quote Machine it first

What to send us

Drawing - 2D and 3D files (STEP, IGES or PDF).
Stainless grade - the family decides the acid system.
Prior operations - machining, welding, blasting or polishing.
Welded zones - these normally need pickling first.
Blind features - so drainage and drying can be planned.
What happens next - assembly or storage, so the hand-over state fits.

All uploads are secure and confidential. Engineers review and reply with a quotation and lead time.