Image Credit: Pavco, Inc.

Passivates are chemicals used to reduce surface reactivity to protect a metal against corrosion. This can be done by depositing an inert coating or inducing the formation of a coherent oxide layer, which is called passivating a surface.

The Passivation Process

Mechanisms for passivates differ based on the chemical makeup of the surface material and passivate, but all function based on the same general method. Surfaces are pretreated in some manner to clean the surface of any additional particles. Passivates are then applied to the surface of a material and react with that material (substrate) to remove corrosion causing impurities and to form a thin protective layer; this layer inherently resists corrosion from certain substances that more readily corrode the substrate.

Design Tip: Proper bath preparation is important for effective passivation. The type and concentration of the passivate along with the bath temperature and time of immersion are all critical to successful treatment. Thus a thorough knowledge of the material types and passivation processes is crucial to achieving the desired results.

Passivation can be considered a separate process from the application of conversion coatings. This is because passivation acts as a cleaning process that allows for oxide formation on a surface, whereas conversion coatings react with the surface to form complex surface films. For example, chromate conversion coating on aluminum utilizes a chromic acid bath with special additives to produce a complex, non-crystalline, non-porous oxide film entrapped with chromates and dichromates.

 

Passivates Selection

Passivates should be selected based on the surface material being addressed, its performance characteristics, and its physical properties.

Surface Material

Passivate selection is essentially a function of the surface or substrate material being addressed. The most commonly treated materials are stainless steel and aluminum alloys. Other materials that can be passivated include zinc, titanium, silicon, and nickel.

  • Stainless steel passivation begins with surface cleaning through degreasing, solvent cleaning, or alkaline soaking. This is followed by immersion in a passivate solution, most typically nitric acid, which removes "free iron" contamination on the surface from machining and fabricating. Free iron sites would otherwise be potential premature corrosion sites. The passivate solution then facilitates the formation of a chromium-rich oxide film on the purified surface which acts as a barrier.

Video explaining a small scale passivation process of stainless steel.

 

Design Tip: It is important to clean the stainless steel workpiece thoroughly of any grease, coolant, or shop debris that may have adhered to the metal surface. Any defects can react with the passivate to form bubbles or other deformities which interfere with passivation. Debris can also cause contamination of the passivate solution which can result in a deteriorated surface finish or incomplete passivation.

 

  • Aluminum alloys typically have little corrosion resistance compared to pure aluminum which forms a corrosion resistant oxide layer naturally on its surface. After surface cleaning, aluminum alloys can be passivated through an anodizing process. Anodizing is an electrochemical process that forms a thin barrier oxide coating on the aluminum as well as a thick oxide coating which makes it useful for electrical insulation and automobile and aerospace corrosion resistant structures.

 

Metal

Concentrated Sulfuric & Nitric Acid

Hydrochloric Acid, Dilute Sulfuric Acid (1)

Acidic Solution

Neutral Solution (2)

Sea Water

Alkaline Solution (3)

Aluminum

Passive

Corrodes

Corrodes

Passive

Passive (Pitting)

Corrodes

Titanium

Passive

Corrodes

Passive

Passive

Passive

Passive

Zinc

Corrodes

Corrodes

Corrodes

Protective Film

Protective Film

Corrodes

Iron

Passive

Corrodes

Corrodes

Corrodes

Corrodes

Passive

Chromium

Passive

Corrodes

Passive

Passive

Passive

Passive

Nickel

Corrodes

(Resistant)

(Resistant)

Passive

Passive (Pitting)

Passive

Lead

Corrodes

Corrodes

Protective Film

Protective Film

Protective Film

Corrodes

Stainless Steel

Passive

Corrodes

Passive

Passive

Passive (Pitting)

Passive

Copper

Corrodes

Stable

Protective Film

Protective Film

Protective Film

Protective Film

Gold

Stable

Stable

Stable

Stable

Stable

Stable

(1) Dissolved oxygen and chlorine degassed, (2) No chlorine ions, (3) Excluding strong alkali such as 40% caustic soda and ammonia (ammonia will form complex salt and dissolve).

Table 1 - Table listing the corrosion resistance of various metals. Table Credit: Misumi Group Inc.

 

Performance Specifications

Performance of passivates can be measured by corrosion resistance time and the type of corrosion or material protection they provide.

  • Corrosion resistance time is given for passivates that provide temporary rather than permanent or semi-permanent resistance to certain types of materials or corrosion. A product's performance may be specified based on the hours of resistance provided.
  • Resisted materials are listed for passivate products that provide protection against specific types of corrosive substances. For example, the nickel fluoride layer formed on nickel metal provides protection against corrosion from fluorine gas.

 

Design Tip: Tests are often performed to evaluate the passivated surface. It is important that the test method be matched to the grade under evaluation. Tests that are too severe or too lenient will not provide accurate results.

 

Physical Properties

Certain physical properties, such as form and color, can also distinguish passivates.

  • Form is the physical state of the passivate. Most are liquids, however some come in pastes or gels. Form determines the method of passivate application. Most passivates are utilized via a bath, but some can be applied by spraying, brushing, swabbing, or electrolytic methods.
  • Colors can also characterize passivates. However, most do not cause a change in the color of the substrate because the layer formed is generally very small.

References

Surface Pretreatment - Chromate Conversion Coatings

Electrochemistry Encyclopedia - Anodizing

Misumi Group Inc. - Surface Finishing, Table 1

Kepco Inc. - Passivating Stainless-Steel Parts

Eastern Plating Company, Inc. - Passivation

Delstar Metal Finishing, Inc. - Passivation