Stainless steel passivation: how to recognize it through testing

Many companies often ask themselves whether the product just made in stainless steel has been correctly passivated or not and especially if the passivation layer is present on all the welding and with what thickness.

Stainless steel workers know very well that a bad passivation will cause a probable corrosion and the consequent loss of solidity and strength in addition to the formation of oxides.

Making sure you have done this process correctly is critical before handing the job over to your customer.

What is passivation technically?

Passivation is the development of a layer of protective chromium oxides on the surface of the stainless steel.

This property, intrinsic in the structure of the metal, is very appreciated because it inhibits the corrosive phenomena that can be activated on the surface of the stainless steel, increasing the useful life of the product and maintaining the mechanical properties unchanged.

Passivation is activated by “natural” processes such as exposure of steel to oxygen in the air. It can be accelerated by chemical/electrochemical processes, such as the immersion in baths of mixtures containing oxidizing acids.

Passivation treatments have the purpose of creating a protective layer on the stainless steel, givin it more resistance to corrosion and maintaining all the qualities and characteristics for which it was chosen.

Now let’s see what solutions exist and the various pros and cons

Which tests allow me to understand if the passivation happened correctly?

There are different methodologies and tests to understand if passivation has been achieved correctly and we can group them into three macro areas:

  1. in the laboratory but “invasive”;
  2. “on-site” but on sight;
  3. precise, “on-site” and non-invasive.

 

In the first two macro areas we find: “Water wetting and drying test” (A380) and “water immersion test” (A967), High humidity test (A380, A967), Ferroxyl test (A380, A967) and Copper sulphate test (A380, A967).

 

The disadvantages of using these types of inspections are mainly:

  • Some require to be carried out in the laboratory with specialized personnel, greatly increasing costs and time;
  • Some, such as Ferroxyl, leave a halo on the stainless steel that then must be removed, lengthening the time and costs;
  • Some use substances that are hazardous to health and even in this case require appropriate places and specialized personnel;
  • They are not able to determine the quality of the passivated layer, which is very important to avoid the formation of future corrosion or halos;
  • If not used in the most appropriate way they risk ruining the artifact or giving false results;

 

As precise and non-invasive tests exist those via cyclic polarization or the Passi Test Plus.

Let’s look in more detail.

Tests using cyclic polarization are carried out exclusively in the laboratory and just by experts in electrochemistry.

It is based on an electrolytic cell in which the sample is immersed in an electrolytic solution.

Through a potentiostat the voltage increases periodically, and the system detects the current change. The current values are very low (micro amperes per square centimetre) and voltages up to 1.6 volts are reached.

This technique determines very precise numerical values of the corrosion resistance of stainless steel.

These values are processed to create a diagram called “polarization curve“.

Thanks to this, it is possible to classify the resistance of the product to corrosion and to understand the degree of protection guaranteed by the oxide.

Certainly, this method, compared to those described above, is far more reliable but at the same time requires highly specialized personnel, time and the need to bring the product in the laboratory (higher costs).

For those looking for a reliable test, but that does not require the need for laboratories with specialized personnel or the need to handle hazardous chemical material for the operator and for the stainless steel, then can use the Passi Test Plus.

Passi Test Plus: accuracy, speed and no risk

Passi Test Plus is an electrochemical device designed by us, able to determine the presence of the passive layer through a value expressed in volts.

If the result is positive then it means that the passivation and also a correct layer have been detected. A negative result indicates that the passivation is not yet complete and that a corrosive attack is possible.

Accurate, simple, portable and fast!

But that’s not all.

The 10 advantages of using the Passi Test Plus:

  • Possibility of evaluating passivation in the workplace;
  • Passivation can be analysed even on complex and hidden surfaces thanks to a probe and a very small test area;
  • The chemical solution does not corrode the material and is not dangerous for the operator. The tested area does not need a subsequent passivation;
  • The control is quantitative and qualitative: the display shows a value expressed in Volt that can be compared and evaluated;
  • Easy to use and carry;
  • It does not require preventive chemical preparations;
  • It does not require specialized personnel;
  • There are no disposal costs;
  • The solution is not sprayed on the surface but is contained within a small pad of absorbent material. In this way you do not consume large quantities of product;
  • Possibility to store the data in the internal memory of the device. In this way, the tests can be recorded in a report that can be attached to the final product to certify the quality of the product.

 

If you want to know more about Passi Test Plus here you can find the link to the page where you can also download the data sheet and send us any questions or requests.

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