Passivation of austenitic and martensitic stainless steels

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Passivation is a chemical process resulting from the formation of a surface layer of a non-friable and homogeneous oxide on the surface of a metal. Aluminum, titanium, cobalt chromium, stainless steels allow the formation of such a passive layer that protects the substrate.

In the case of  stainless steels, i.e. containing at least 12.5% chromium, this process can be controlled by immersing the parts in phosphoric acid and more commonly nitric or citric acid, following different temperature, time and concentration parameters such as those presented in ASTM A967M (Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts) standard.

However, depending on the different types of stainless steels and their applications, this passivation can be more or less complex. 316 L steel, of austenitic type, due to its highly alloyed composition (17% Chromium, 12% Nickel, 2% Molybdenum) is passivated relatively easily.

The same is not true for martensitic steels, or hardened by precipitation. To allow, during quenching, the sudden change of the crystal structure that will bring them their hardness, these steels cannot contain nickel, which would maintain the austenitic structure at room temperature.

The absence of nickel leads to a lesser dissolution of chromium in the steel alloy (Iron + Carbon). However, it is the oxidation of chromium that constitutes the passive layer. The 400 series steels, martensitic (410, 420, etc.), rarely exceed 13% chromium content, making these steels the least resistant to corrosion. Following the laser marking operations, mandatory for any medical devices, the passive layer is altered, especially if the marking is done with fiber lasers, commonly used and focusing sufficient energy to pass through the passive layer.

Thanks to the know-how of our R&D team and in collaboration with Cetim, NGL has been able to develop an efficient and controlled passivation process on austenitic and martensitic steels, based on citric acid, without CMR compounds, whose composition complies with the requirements of regulation EU2017/745. The results of our tests that we present to you below speak for themselves.

We are waiting for you at orthomanufacture to present our passivation process, as well as our cleaning range.


NGL process according to the recommendations of ASTM A967

The NGL passivation process was tested on several materials and positive  results were obtained on them: CrCo, XM16, 17-4PH, TA6V, 316L.

However, the passivation of X20Cr13 (Z20) and X30Cr13 (Z30) steels proved more difficult, especially those parts that had undergone laser marking. It was necessary to test different methods of surface preparation before obtaining a passive layer thick and homogeneous enough to withstand corrosion resistance tests.

Before passivation, the parts were cleaned with the Galvex 20.01 already validated in the medical. The contaminations were cutting oil, abrasive residues  and fingerprints. After treatment, a first visual inspection (naked eye + microscope) was carried out to check the general appearance of the parts and the state of the laser marking (visible reading of characters, corrosion,…).

The following results were achieved:

  • Overall appearance: after washing, all contaminations were eliminated without attacking the materials (Z20 and Z30). The parts are not corroded, shiny, without chemical residues or other traces of rinsing.
  • Laser marking: The process described above gives surfaces and laser markings without corrosion bites or halos (polished and micro-balled faces). All characters are readable, bright and not discolored.


In addition to these tests, the following corrosion resistance tests were carried out according to the recommendations of the CRITT:

The martensitic and austenitic steel parts passivated by the Decomet meet the requirements of all these tests.