Sani Marc Group’s virtual magazine

Passivation – Natural Protection for Stainless Steel

Passivation on stainless steel

 

Protection naturelle de l'acier inoxidableNATURAL PROTECTION FOR STAINLESS STEEL

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

Oxygene on stainless steel

 

 

 

 

 

 

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.
    corossion

    Contaminated iron filings on equipment. The ideal scenario for serious corrosion
    problems to occur

 

 

 

 

 

 

  • cleaning_soloution_runoff

    Corrosion caused by cleaning solution runoff that has repeatedly dried on the surface. Note that corrosion has occurred where the solution has run off.

    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)
  • corosion_tank

    Corrosion of a tank possibly caused by its continuous use (no self-passivation in air), poor choice of products

    Overused equipment (steel does not have enough time for self-passivation)

 

 

 

 

 

  • ​Corrosive water supply
    corrosive_water_supply

    Water supply reservoir with noticeable corrosion; possibly due to the evaporation of hypochlorite.

 

 

 

 

 

 

device_measure_passivation

Device for measuring the passivation layer of stainless steel.

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
particular needs.

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.

Table 1. Chrome to iron ratio obtained with various passivation techniques.
Passivation TechniqueCr/Fe Ratio
Cleaning only1.2
Cleaning + phosphoric acid1.5
Cleaning + nitric acid1.75
Cleaning + citric acid2.0

 

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.

surface_rust

Surface rust caused by metal dust following work on nearby equipment.

metal_dust_collected

Metal dust collected from recently machined tanks.

 

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.

plate_heat_exchanger

Plate heat exchanger with severe corrosion. Not treatable with conventional acid treatment

cross_sectional_view

Cross-sectional view of the corrosion layer through electron microscopy.

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.

Daniel Allard
Chemist / Chemistry Support Manager

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