Passivation recognition test
Passi Test Plus: electrochemical test to recognize of passivation on stainless steels
Passivation is the development of a layer of protective oxides on the metal surface. This property, intrinsic in the metal structure, is highly appreciated because it inhibits the corrosive phenomena that can be activated on the metal surface, increasing the useful life of the product and maintaining the unaltered mechanical properties.
Passivation can be accelerated by “natural” processes, such as the exposure of steel to oxygen, or with chemical / electrochemical processes, such as immersion in baths of mixtures containing oxidizing acids.
When the steel has undergone a treatment of:
It is necessary to take into account the time necessary for the formation of the passive layer of stainless steel.
Many companies, after having manipulated the stainless steel, wonder if the passive layer is present on the surface of the stainless steel before delivering it to the final customer. This question is vitally important for two reasons:
- Provide the customer with a high quality product that supports the requested application;
- Having the guarantee to provide a product that should corrode in certain environments should focus again on the choice of material or find a way to reduce the aggressiveness of the surrounding environment.
Passivation treatments provide a protective layer that puts stainless steel under conditions of its maximum corrosion resistance in a given environment. In other words, there is a limitation of corrosion resistance that depends on the nature of stainless steel, for which even the best passivation treatment on a certain stainless steel does not guarantee total protection for certain applications.
Typical example is in the field of petrochemicals. The environment is very aggressive and therefore the choice of material to be used to build a plant is very important. The choice between AISI 304 or AISI 316, rather than nickel alloys, generates very important effects in the field of corrosion. Even if stainless steel received the best passivation treatment, the petrochemical environment could degrade the material very quickly and not provided for by the initial design phase, with an exponential increase in maintenance costs.
Institutions (ASME) which regulate the passivation treatment state declare that there are no universally accepted tests to guarantee that a component or system has been passivated or is in a passive condition. To help the client recognize passivation, several tests have been developed that can be divided into two groups: coarse inspection and precision inspection.
“Water wetting and drying test” (A380) and “water immersion test” (A967): this test can be performed on the working field simply by immersing the piece in a saline solution from 3 to 7%, rinsing with distilled water and drying it to air. The visual evaluation of iron dust stains and any traces of pitting corrosion does not define a quantitative control of the passive layer.
High umidity test (A380, A967): This test is carried out only in the laboratory on small samples, not suitable for performing field tests. The samples are immersed in an environment rich in acetone or methyl alcohol and then dried in an inert atmosphere. At this point the samples will be tested in an environment with 97% humidity at 37 ° C for a user-defined time (minimum 24 hours). Evaluation is purely visual. At the end of the test it could trigger some corrosive processes due to ferrous contamination.
High humidity test
Ferroxyl test (A380, A967): this test can be carried out on the working field by preparing a chemical solution based on distilled water, nitric acid and potassium ferrocyanide at the time of the test. The solution is sprayed onto the sample surface. After 15 seconds the appearance of bluish spots causes the presence of ferrous contamination on the surface of the stainless steel. This test does not determine the positive effects of the passive layer. The chemical solution is dangerous for the operator and the right amount must be produced in order to limit the costs of disposal and production. The solution must be applied by a qualified staff to avoid irreversibly damaging the stainless steel surface.
Copper sulfate test (A380, A967): Test that can only be done in the laboratory. The test involves the preparation of a chemical solution based on sulfuric acid, copper sulphate and distilled water. The solution is applied several times on a sacrificial sample and is expected for a period of 6 minutes. This test is only visual and can detect only the iron on the surface, it has difficulty in detecting the iron embedded in the surface of the stainless steel.
Copper sulfate test
Cyclical polarization: Laboratory test used only by experts in electrochemistry based on an electrolytic cell in which the sample is immersed in an electrolyte solution. Through a potentiostat, the voltage is periodically increased and the system detects the current variation. The current values are very low (micro Ampere on square centimeter) and voltages up to 1.6 Volt 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 a “polarization curve”.
Curva di polarizzazione tipica dell’acciaio inox
The figure shows the theoretical polarization curve of a stainless steel in an environment containing chlorides. In particular, from the theoretical curve it is possible to obtain some reference values, useful for classifying on the basis of the corrosion resistance different steels or subjected to different surface treatments. In particular:
Ecorr =represents the equilibrium potential (under the applied experimental conditions) that ideally separates anodic and cathodic behavior of the material; for potentials inferior to Ecorr the material will be protected from corrosion (cathodic behavior) while for potentials higher than Ecorr the material will be active, oxidizing (anodic behavior);
Epit = it represents the potential starting from which the protection provided by the passive layer present on the material (chromium oxide) is lost due to phenomena of trans-passivation (oxide failure) or pitting (localized breakdown of the oxide); for the tests under examination (in the presence of chlorides) there will always be a localized breakdown of the oxide, so we will always refer to pitting; the greater the pitting potential, the greater the resistance of the material to this localized corrosion phenomenon;
Ipass media = average passivation current, is the average value assumed by the corrosion current in the passivation interval (ie the potential range within which the material is protected by a stable passivating oxide); the lower the average passivation current, the greater the corrosion protection guaranteed by the passivating oxide.
Passi Test Plus: The Passi test plus is an electrochemical device designed by Nitty Gritty able to determine the presence of the passive layer through a number expressed in volts. The potential (V) determines the nobility of a material, ie its resistance to corrosion compared to another material. For this reason, if the potential is positive then the device has detected the presence of the passive layer, otherwise, if negative, there could be the corrosive attack from the surrounding environment.
The advantages compared to the previous test are:
- Possibility of evaluating passivation at the workplace;
- Passivation can also be analyzed 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: the display shows a number expressed in Volts that can be compared and evaluated;
- Easy to use and transport. It does not require preventive chemical preparations and specialized personnel;
- The chemical solution inside the probe does not degrade and does not expire. In this way the costs of disposal and management of the liquid are zeroed;
- The solution is not sprayed on the surface but is contained within a small pad of absorbent material. In this way large quantities of product are not consumed;
- Possibility of storing the data in the internal memory of the device. In this way the tests carried out can be filed in a report which can be attached to the final product to certify the quality of the product.
Passi Test Plus
Passi Test Plus Report