In the case of stainless steel it's a subtractive process rather than additive one using a heated nitric acid solution (some processes use concentrated citric acid instead) to dissolve elemental iron out of the surface of the part leaving a surface matrix of chrome and other alloys. Sometimes other chemicals are added to the solution.
Either during the passivation process or afterwards with a secondary treatment, the exposed chrome matrix is converted to a thin chromic oxide film on the surface of the part which provides corrosion resistance.
There are a number of varieties of stainless steels: 200, 300, 400, 500 and 600 series plus a number of other types of specialty stainless steels. Stainless steel is a misnomer. The term refers to alloy steels that have alloying metals added to resist corrosion.
The most commonly used types are the 300 series stainlesses like 302, 303, 304. 316 and so on (plus similar but less common 200 series).
They are referred to as 18/8 in Europe and other locations indicating ~18% chrome and ~8% nickel alloy content. These are also sometimes called "inox" or "inoxidable" meaning resistance to oxidation. These alloys have reasonably high resistance to corrosion but are "soft".
300 series stainless steels can only be hardened though cold working processes and the maximum hardness is ~46Rc making parts made of these alloys unsuitable for high wear or pressure applications such as ball bearings.
Some 400 series stainless steels can be hardened through a heat treating process by heating to a very high temperature and quenching. 440C is one of those stainless steels. It's more like a tool steel than a common stainless such as 304 and can be hardened to 66Rc+. It's used in knifes and applications requiring high hardness and wear resistance with some protection to mildly corrosive materials due to the 20% chrome content.
440c could be used for ball bearings but it's far more expensive than the common 52100 chrome alloy steel used in most types of bearings and doesn't add much utility for ball bearings found in bikes.
Passivating a highly polished surface such as found on precision ground ball bearings is counter productive because it destroys the smooth polished surface that reduces friction and can change the dimensions too.
Yes high precision 300 series and 440c balls are available but they are not intended as replacements for common ball bearing applications! We deal with a company that makes and/or sells precision sized balls made of everything from plastic to tungsten carbide, ruby and sapphire.
Frequently these precision balls are used for processes that have nothing to do with friction reducing ball bearings such as measuring hole sizes or "ballizing" which is sizing existing holes in parts by pressing hardened precision ground balls through the part.
Standard grade 25 chrome steel balls are still the best choice for almost all cycling applications!
Chas. Colerich Oakland, CA USA
Jerome & Elizabeth Moos wrote:
> Actually. passivation has a broader meaning as reducing the chemical reactivity of just about any substance, but often metals. Many chemical process catalysts are made of silica with certain metals added. But other metals may be deposited on the catalyst by the material being processed. Passivators work to deactivate the unwanted metals to prevent them from interferring with the desired reaction or from causing unwanted reactions. There is a whole large industry which makes industrial catalysts, and most companies also sell additives, which include paassivators to keep materials from interferring with the working of their catalysts.
> So a broad definition of passivation might be adding stuff to the stuff you already have to keep the stuff you already have from doing stuff you don't want it to do. How's that for a technical definition?
> Jerry Moos
> Big Spring, Texas, USA
>> From: David Kulcinski
>> Subject: Re: [CR] Metric bearings on bikes?
>> Date: Tuesday, December 22, 2009, 2:39 PM
>> Passivation is the process of
>> removing iron molecules from the surface of stainless steel
>> so that it doesn't rust as quickly. It is, usually, based
>> on acetic acid.
>> Thank you,