Passivation of Stainless Steel

passivated partsPassivation is the chemical treatment of a stainless steel surface to remove any “free-iron” or other contaminants from the surface of the metal while at the same time promoting the formation of a thin, dense oxide and corrosion protective barrier.

The Passivation process designed to remove foreign metals from the surface of stainless and corrosion resistant steels by immersing a cleaned and descaled steel part into a solution of citric or nitric acid and oxidizing salts. Passivation dissolves exposed surface iron and produces a transparent film of corrosion-resistant chromium oxide. It does not change the appearance of the base metal.

Passivation of Stainless Steel – AMF’s Capabilities

Passivation Specifications:
  • ASTM A967
  • AMS 2700
  • QQ-P-35
  • ASTM 380
  • ASTM F86
  • AMS 2700
  • ISO 16048
  • Most Company Specifications
Types of Passivation:
  • Precision Medical-Grade Passivation
  • Commercial Passivation
  • A-A-A (Alkaline-Acid-Alkaline) Passivation
  • Stainless Steel Grades Passivated:
  • Austenitic Grades (All 200 & 300 series)
  • Ferritic Grades (Part of 400 series and low carbon alloys)
  • Martensitic Grades (Part of 400 series and higher carbon alloys)
Passivation Methods:
  • Nitric Acid Passivation
  • Citric Acid Passivation
  • CitriSurf Passivation
  • A-A-A (Alkaline-Acid-Alkaline) Passivation (Citric or Nitric)

Passivation to ASTM A967

  • Five nitric and citric methods are listed as follows:
  • Nitric 1: 20-25 v% Nitric Acid, 2.5 w% Sodium Dichromate, 120-130F, 20 Mins minimum
  • Nitric 2: 20-45 v% Nitric Acid, 70-90F, 30 Mins minimum
  • Nitric 3: 20-25 v% Nitric Acid, 120-140F, 20 Mins minimum
  • Nitric 4: 45-55 v% Nitric Acid, 120-130F, 30 Mins minimum
  • Nitric 5: Other combinations of temperature, time, and acid with or without accelerants, inhibitors or proprietary solutions capable of producing parts that pass the specified test requirements
  • Citric 1: 4-10 w% Citric Acid, 140-160F, 4 Mins minimum
  • Citric 2: 4-10 w% Citric Acid, 120-140F, 10 Mins minimum
  • Citric 3: 4-10 w% Citric Acid, 70-120F, 20 Mins minimum
  • Citric 4: Other combinations of temperature time and concentration of citric acid with or without chemicals to enhance cleaning, accelerants or inhibitors capable of producing parts that pass the specified test requirements.
  • Citric 5:  Other combinations of temperature time and concentration of citric acid with or without chemicals to enhance cleaning, accelerants or inhibitors capable of producing parts that pass the specified test requirements.  Immersion bath to be controlled at pH of 1.8-2.2
  • Five testing methods to validate passivation services are provided as follows:
  • Practice A – Water Immersion Test
  • Practice B – High Humidity Test

Passivation to AMS 2700

  • AMS 2700 covers passivation of stainless steel to both nitric and citric method
  • Method 1 – Passivation in Nitric Acid (see types below)
  • Method 2- Passivation in Citric Acid (4-10w% Citric Acid, various temperature/time ratios are offered and additional wetting agents and inhibitors are acceptable)
  • The following types may be specified for Method 1:
  • Type 1 – Low temperature nitric acid with sodium dichromate (20-25v% nitric, 2-3w% sodium dichromate, 70-90F, 30 mins min)
  • Type 2 – Medium temperature nitric acid with sodium dichromate (20-25v% nitric, 2-3w% sodium dichromate, 120-130F, 20 mins min)
  • Type 3 – High temperature nitric acid with sodium dichromate (20-25v% nitric, 2-3w% sodium dichromate, 145-155F, 10 mins min)
  • Type 4 – 40v% nitric acid for free machining steels (38-42v% nitric, 2-3w% sodium dichromate, 70-120F, 30 mins min)
  • Type 5 – Anodic, for high carbon martensitic steels (20-25v% nitric, 2-3w% sodium dichromate, 70-90F, 2 mins min, part anodic at 3-5 volts)
  • Type 6 – Low temperature nitric acid (25-45v% nitric, 70-90F, 30 mins min)
  • Type 7 – Medium temperature nitric acid (20-25v% nitric, 120-140F, 20 mins min)
  • Type 8 – Medium temperature, high nitric acid concentration (45-55v% nitric, 120-130F, 30 mins min)
  • 3.1.3 Method 2 – Passivation in Citric Acid
  • 3.1.3.1 Bath Composition
  • Parts shall be immersed in an aqueous solution of 4 to 10 percent citric acid, with additional wetting agents and inhibitors as applicable.
  • 3.1.3.2.1 Temperature
  • Bath temperature shall be 70 to 160F (21 to 71C) with an immersion time of not less than 4 minutes for baths operating over 140F (60C), not less than 10 minutes for baths operating in the    120 to 140F (49 to 60C) range, not less than 20 minutes for baths operating in the range of 100 to 119F (38 to 48C) or not less than 30 minutes for baths operating below 100F (38C).
  • 4-10w% Citric Acid, 140-160F, 4 Mins Min
  • 4-10w% Citric Acid, 120-140F, 10 Mins Minimum
  • 4-10w% Citric Acid, 100-119F, 20 Mins Minimum
  • 4-10w% Citric Acid, 70-100F, 30 Mins Minimum
  • Where no type is specified, the processor may use any of the listed types that meet the requirements given within AMS 2700.
  • The following Classes are provided for testing within AMS 2700 (if no class is defined, Class 2 shall apply):
  • Class 1 – Testing not defined or as specified by the customer, test per 4.3.1 sampling plan
  • Class 2 – Testing shall be one part per lot
  • Class 3 – Frequency of testing shall be on a periodic basis
  • Class 4 – Frequency of testing defined by 4.3.4 sampling plan (testing previously defined by QQ-P-35)
  • Various corrosion resistance tests are defined within AMS 2700.  However, it is noted that certain alloys such as high carbon alloys with 0.85% carbon or more (such as 440C) are exempt from various testing requirements due to false positives that can result.  Parts shall meet one or more of the following tests:
  • Humidity Test
  • Water Immersion
  • Copper Sulfate Test
  • Salt Spray Test
  • Table 4 within AMS 2700 provides a comprehensive summary of both nitric and citric acid methods that can be applied as a function of alloy.  This table is more comprehensive than those listed in either ASTM A967 or QQ-P-35.

Passivation to QQ-P-35

  • QQ-P-35 does not cover citric acid passivation services.  Four active nitric acid passivation services are covered as follows (four inactive methods have been withdrawn):
  • Type I – Withdrawn
  • Type II – 20-25 v% Nitric, 2-2.5 w% Sodium Dichromate, 120-130F, 20 Mins minimum
  • Type III – Withdrawn
  • Type IV – Withdrawn
  • Type V – Withdrawn
  • Type VI – 25-45 v% Nitric, 70-90F, 30 Mins minimum
  • Type VII – 20-25 v% Nitric, 120-150F, 20 Mins minimum
  • Type VIII – 45-55 v% Nitric, 120-130F, 30 Mins minimum
  • Four testing methods to validate passivation services are provided as follows:
  • 4.4.1.1 – Water Immersion Test
  • 4.4.1.2 – High Humidity Test
  • 4.4.2.1 – Salt Spray Test
  • 4.4.2.2 Copper Sulfate Test
  • A table of recommended nitric acid passivation services is provided in the Appendix that correlate Type II, VI, VII & VIII methods to the specific stainless steel alloy grade.