Passivation Of Stainless Steel

Introduction

There are many industries that demand high-purity systems in which they can manufacture their products (pharma, food, cosmetics, semi-conductor, etc.) or whose processes are carried out in hostile environments and need to be assured that their equipment is up to the task (nuclear, oil & gas, etc.). Contamination in any of these industries can result in serious consequences. The product can be affected and/or the equipment can be damaged resulting in downtime and substantial unexpected costs. In extreme cases, the equipment can fail catastrophically, leading to environmental and safety issues. 

Contamination in stainless steel can be introduced in multiple ways and at any time of its life-cycle.

Beginning with the melting phase, in which low-grade recycled ingredients are melted down and then ‘corrected’ with additions of nickel, molybdenum or other elements. There is often a significant difference in the quality of the base material, depending upon where it is sourced from.

Cutting and fabrication processes can introduce various types of debris, including ferrous material, into the stainless steel. Then there are the well-documented issues associated with welding - in which the heat-affected zone damages the passive layer, depletes the chromium concentration in that region and leaves a porous scale where corrosion can initiate at a later date.

Surface finishing will of course (in most cases) improve the chances of a corrosion-free product, but they also create an opportunity for contamination. Usually, and depending largely on the environment the stainless goes into service in, there will be no issues. Equipment used in high-purity systems however are very vulnerable and extra precautions are essential. Iron or aluminium oxide abrasive media is commonly used in mechanical polishing processes. These materials are very hard and can be embedded into the equipment surface. As these metals are less noble than stainless steel, galvanic corrosion will occur, with the embedded materials eventually corroding away. This will then leave an inclusion or ‘pit’ which can lead to bigger issues. Embedded iron is a notable problem as this will prevent the passive layer from re-forming whilst ever it is present.

Finally, contamination can happen during installation. New installations are often construction-sites and are far from the clean-room environment that will house the finished system. Concrete dust, wood particles, ferrous debris and a multitude of other unwanted airborne elements can enter the system. All it needs is an momentary lapse of control in which a pipe-connection or vessel flange is left uncovered at the wrong time and foreign material will settle on the surface and either directly initiate corrosion, or inhibit the passive layer from fully forming.

It is vital then that all contamination is fully removed before the system is put into service. The only way to do this, is to carry out a thorough chemical cleaning operation which incorporates degreasing (to remove organic residues) and a passivation stage - to remove everything else, but primarily, unwanted metals.

Passivation

Chemical passivation is the process in which the stainless steel surface is exposed to (usually) an acid in order to remove any contamination and increase the chromium to iron ratio, leaving the surface complete inert and in optimal condition for forming a dense chrome-oxide layer that gives stainless steel its corrosion resistant qualities.

Process

Degreasing:

The degreasing stage will remove all organic contaminants such as fats and oils along with others. On systems that have already been in service, this may include dried residues from synthesis, distillation residues, ointments and creams, etc.

Inox Passivation use an alkaline detergent that contains complexing and chelating agents. This is circulated at a relatively low concentration (0.5 - 2%) at a high temperature (60-70ºC). It is surfactant-free, chlorine-free and suitable for validated cleaning. 

Passivation:

There are a couple of options for passivation, and the chemistry should be chosen after reviewing the system to be treated.

• Nitric acid based passivating solutions are highly effective at removing iron particles. Nitric is corrosive and will dissolve ferrous contaminants in the system very quickly. You can use different concentrations of nitric acid to achieve the desired result – but it invariably becomes a trade-off between concentration, temperature and contact time. 

  Nitric acid can only be used on austenitic and duplex grade stainless steels. 

• Citric acid based products work in a slightly different way. After dissolving the contaminant, they ‘chelate’ the ion, effectively bonding to it. The contaminant (dissolved iron for example) cannot precipitate back out and so you can be assured that everything is removed during the flushing stage. (In the case of nitric acid, it is argued that dissolved ferric iron will precipitate out of solution at pH 3.5. If the flushing stage is interrupted, or performed at very slow flow-rates, iron could recontaminate the surface).

Citric acid can be used safely on all stainless grades. On ferritic and martensitic grades, it can be combined with hydrogen peroxide to increase its effectiveness. 

• Hydrogen peroxide based solutions are highly oxidising and effectively ‘force’ the chromium-oxide layer to reform. Hydrogen peroxide on its own is not a very effective cleaner. If the surface is not already spotlessly clean, it cannot be considered as effective as the more conventional nitric or citric acid alternatives. That said, it is the perfect option for following our pH neutral derouging process – which will remove all traces of iron contamination prior to passivation/oxidisation.

Testing

Throughout the passivation process, the circulating chemicals are periodically analysed to determine the acidity and dissolved iron concentration using our portable laboratory equipment. When the iron concentration % remains constant over three consecutive readings, the process is complete. Be wary of a specification that gives a fixed contact period. Just because it says "circulate for 1 hour", this doesn't necessarily mean that this will be sufficient to remove all of the contamination.

Additional inspections can include the ferroxyl test, water immersion test and/or AES (Auger Electron Spectroscopy) to determine the chrome/iron ratio on the metal surface. Passivity of the stainless steel surface can also be measured with an oxyliser – passivity meter. These tests are normally carried out on stainless steel coupons as they could contaminate the now clean system, or physically cannot be carried out in-situ.

Summary

It is essential that your stainless steel systems and equipment are correctly cleaned and passivated before going into service. The cost of pre-commissioning passivation is much less than the costs associated with replacing corroded parts or failed product batches should the system be put into service without carrying it out. Routine cleaning and re-passivation should also be a part of your planned preventative maintenance. Frequency will vary depending upon the use and design, but needless-to-say, high-purity water systems in particular are vulnerable to corrosion (in the form of rouge) and over time, will tarnish. A scheduled passivation process will keep the system in optimal condition.