Stainless steel has long been a choice material for manufacturing various types of food processing equipment. It’s popular for several reasons:
• It is highly resistant to impacts, strain, wear, abrasion and erosion.
• It is malleable, easy to weld and easy to machine.
• It is highly resistant to corrosion and chemicals.
• It is resistant to extreme temperatures and thermal shocks.
• Its smooth, nonporous surface prevents the adhesion of food and reduces the adherence of biofilms.
• It does not contaminate food or alter its organoleptic properties.
• It has a nice appearance.
Stainless steel’s high resistance is due to its ability to form a selfhealing protective layer. When stainless steel becomes damaged, the exposed surface reacts with oxygen in the air or with water to re-form a protective layer
Despite this ability to form a protective barrier, some conditions may gradually or systematically erode this layer and cause surfaces to corrode. Under such conditions, damage can occur very rapidly. The most common causes are:
- Equipment modification: When steel is welded, scrubbed with a wire brush, sanded, etc.
When the metal is exposed to incompatible compounds or adverse conditions: Cleaning steps omitted, incompatible products, excessive operating temperatures, products drying on the surfaces, insufficient pre-rinsing or washing, inadequate phase cut-off, concentration of sequestrant too low, wrong choice of additive in alkaline solutions
- Incompatible production equipment (brine, for example)
Overused equipment (steel does not have enough time for self-passivation)
- Corrosive water supply
For all these reasons, it is important to regularly inspect equipment so that you can detect the early signs of corrosion and protect your investment. A good visual inspection will quickly identify any early warning signs. There are also devices for measuring the state of the passive layer on stainless steel.
It is also essential to ensure that the cleaning products and methods being used have minimal impact on surfaces.
When the environment prevents an adequate protective layer from forming, surfaces should be treated to quickly create this protective oxide layer on the surface. This is called passivation.
The goal of passivation is not to eliminate corrosion; corrosion must be removed prior to this treatment.
WHEN SHOULD YOU PASSIVATE?
• Before commissioning equipment
• When making modifications to the system (welding, for example)
• When the system is contaminated (by iron dust, for example)
• Upon the first signs of corrosion
• When the protective layer is gone
• As a preventive measure
There are various passivation techniques that can be used depending on the equipment requiring treatment (tanks, pipes, outer vertical surfaces). A specialist can help you determine which methods are best for your
Generally, if metal dust is suspected on the surface, it must be removed, with manual cleaning if possible. Using cationic products tends to offer the best results.
It is important to ensure that the surfaces to be treated are free of organic deposits (oil & grease, protein, starch) as well as inorganic deposits (scale, silicate, rust) prior to starting passivation. The metal to be treated must therefore first undergo alkaline and acid cleaning to ensure it is free of deposits.
Various products can be used for passivation. Phosphoric acid was originally used for this type of treatment, but has since been replaced by nitric acid, which offers superior performance. Recent studies have shown that citric acid provides better protection by increasing the chrome to iron ratio on the surface. The higher this ratio, the greater the corrosion protection.
|Passivation Technique||Cr/Fe Ratio|
|Cleaning + phosphoric acid||1.5|
|Cleaning + nitric acid||1.75|
|Cleaning + citric acid||2.0|
Citric acid is also much safer to handle and has a better environmental profile. While passivation is an excellent investment in maintaining your equipment, it is nonetheless an operation that requires additional efforts.
It would be unfortunate to complete this treatment only to realize after the fact that mistakes have been made. For example, there is no point in doing passivation if work is being done nearby because the surfaces will most likely become contaminated very quickly. Proper planning and monitoring are therefore key to successful passivation.
Just as in most areas, prevention is your best strategy. If you choose to ignore the warning signs, you could end up with severe corrosion problems that cannot be treated with traditional techniques.
Fortunately, there are unconventional restoration techniques that can be used to correct these chronic problems. Clean Steel technology is one of them. We will discuss this in an upcoming educational bulletin.
Chemist / Chemistry Support Manager