Passivation of Stainless Steel Services

Passivation of Stainless Steel Services ASTM A967, AMS 2700 & QQ-P-35

Passivation of stainless steelAdvanced Plating Technologies is an industry leader in passivation of stainless steel services to ASTM A967, AMS 2700 and QQ-P-35 specifications, employing both citric and nitric acid methods.  APT provides both competitive high-volume commercial stainless steel passivation as well as precision medical passivation of titanium and other medical-grade alloys including cobalt chromium, MP35N and 316LVM, utilizing ultrasonic systems for demanding applications within the medical and dental industries.  APT provides passivation of stainless steel across various sectors including the medical, dental, aerospace, power transmission/distribution, heavy equipment and petrol chemical industries.

Advanced Plating Technologies offers passivation of stainless steel to all methods listed within ASTM A967, AMS 2700 and QQ-P-35.  In addition, APT can meet the requirements of any company-specific nitric or citric acid stainless steel passivation methods.  APT also provides passivation of stainless steel to the Carpenter A-A-A process to passivate difficult stainless steel alloys including high-sulfur free machining 303 or 416 alloys as well as high chromium alloys such as 440 stainless (reference:  A-A-A Passivation Methods article in our Metal Finishing White Papers section).  Inhibited passivation solutions are available to maintain bright surfaces of stainless components such as machined faces and centerless ground 303 or 416 stainless steel shafts.

Passivation of stainless steelUnfortunately, very few part blueprints indicate the specific stainless steel passivation method to be used.  This opens the door for “brand-x” job shops to use any convenient method they have available resulting in a part that has been passivated but is not truly passive (reference: My passivated “stainless” steel product is exhibiting corrosion, how can this be prevented? in our Plating Topics section).

APT’s extensive experience in passivation of stainless steel ensures that the correct passivation method will be selected to match the specific stainless steel grade.  Advanced Plating Technologies provides full in-house testing services to certify the performance of our stainless steel passivation including high humidity, salt spray per ASTM B117, potassium ferricyanide and copper sulfate.

Passivation of stainless steelStainless steel or corrosion-resistant steel (CRES) is defined as a steel alloy with a minimum of 11% chromium content by weight.  Many alloys of stainless steel include substantial nickel to further enhance the corrosion resistance.  In general, stainless steel is broken down into three main groups based upon the alloying elements and compositions as follows: 1) Austenitic Grades (all 200 & 300-series), 2) Ferritic Grades (part of the 400-series, low carbon alloys) and 3) Martensitic Grades (part of the 400-series, higher carbon alloys).

The reason for the stain-less nature of stainless steel is due to the formation of a thin, adherent and passive (nonreactive) film of nickel & chromium oxides that forms a barrier to prevent further surface corrosion of the product.  This is in stark contrast to iron oxide (red rust) that forms on plain carbon steel products.  Iron oxide is a loose, scaly oxide that easily falls away to allow the formation of additional iron oxide, thereby perpetuating the corrosion reaction.

When stainless steel products are manufactured, free iron is transferred to the surface of the material from the steel cutting, stamping and forming tools used in the manufacturing process.  Free iron can also be imparted on the surface by polishing or blasting operations that utilize the same polish or blast media between both mild steel and corrosion resistant steel grades.  Free iron readily oxidizes, forming visible rust on the surface of the product.  Passivation of stainless steel is a chemical treatment with a specific acid formulation that removes free-iron or other surface contamination from the stainless steel while simultaneously promoting the formation of a passive chromium/nickel oxide layer to act as a barrier to further corrosion.

It is important to note that stainless steel is corrosion resistant but not corrosion proof.  The degree of corrosion resistance of a stainless steel alloy is a function of the alloying composition, heat treatment, internal stresses and passivation treatment.  An example of this phenomenon is 303 free-machining stainless steel which has notably less corrosion resistance than 304 stainless steel due to the higher concentration of sulfur and phosphorous that imparts the desired machinability of the 303 grade.  As a general rule, the higher the nickel and chromium content in the alloy, the more corrosion resistance it will have.

The most common stainless steel passivation specifications certified by Advanced Plating Technologies are ASTM A967, AMS 2700 and QQ-P-35.  APT can also certify passivation of stainless steel to ASTM A380, ASTM F86, AMS 2700C, ISO 16048 as well as most company-specific passivation specifications.  A summary of stainless steel passivation techniques per the common ASTM, AMS and Mil specs is as follows:

Passivation of Stainless Steel 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

Passivation of stainless steelCitric 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
Practice C – Salt Spray Test
Practice D – Copper Sulfate Test
Practice E – Potassium Ferricyanide-Nitric Acid Test

A table of recommended nitric acid passivation methods is provided in the Appendix that correlate Nitric 1 through 5 methods to the specific stainless steel alloy grade.  No such reference is provided in the specification for Citric 1 through 5 methods.

Passivation of Stainless Steel to AMS 2700

AMS 2700 covers passivation of stainless steel to both nitric and citric metho

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)

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 of Stainless Steel 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.

Advanced Plating Technologies provides expertise in passivation of stainless steel to fulfill even the most demanding applications.  From prototyping and PPAP qualification to full production processing, it is APT’s dedicated effort to exceed your expectations.  The following points highlight why Advanced Plating is a leader in passivation of stainless steel:

  • Precision Passivation: Advanced Plating Technologies has worked with numerous manufacturers to solve existing stainless steel passivation problems where stringent cleanliness, appearance and corrosion performance was required. We have the ability to use ultrasonic generators in our passivation services as well as in the final rinsing steps if required. These systems allow proper passivation of complex geometries or even very small and intricate parts such as those used in the medical and dental industries.
  • A-A-A Method of Passivation:  Advanced Plating Technologies has a line built from the ground-up to provide the A-A-A (alkaline-acid-alkaline) method of stainless steel passivation developed for passivation of 303 and 416 free machining grades of stainless steel.
  • Passivation of stainless steelCommercial Line: In addition to our precision passivation services Advanced Plating Technologies offers extremely competitive commercial grade passivation on a high volume production line. This guarantees the most competitive pricing in the industry on high volume commercial grade components.
  • Grade-Specific Passivation: Many finishing companies passivate all stainless steels the same with a false methodology of dip and ship.  At Advanced Plating Technologies we are experts in matching the stainless steel passivation method to the specific material grade.  This results in parts that are passivated correctly with results that can be validated and certified.
  • Any Specification Anytime: If you need to certify passivation of stainless steel to a company specification that is “outside the normal” we can help. We can set up specific acid and inhibitor chemistries to service you
  • RoHS and REACH Compliance:  Advanced Plating Technologies is at the forefront of environmental compliance and can certify many of our passivation services as needed to the current RoHS, ELV and REACH regulations.
  • Competitive Pricing:  Advanced Plating Technologies has been providing quality passivation services for over sixty years.  Competitive pricing in stainless steel passivation is paramount to gaining and maintaining our customers in today’s competitive global marketplace.
  • Lean Lead Times:  With inventories leaner than ever, rapid turn-around is a foundational company value to ensure the service our customers need in passivation of stainless steel.
  • Passivation of stainless steelExtensive in House Analytical and Quality Lab Services:  SPC is employed in all wet-lab work surrounding our stainless steel passivation to ensure baths are analyzed and adjusted at optimum frequencies to maintain operation between critical upper and lower control points.  Our QC lab employs the latest technologies to inspect your parts and will work to your requirements to provide testing as needed, and can certify our passivation of stainless steel to any of the test methods outlined in the ASTM, AMS or MIL specs.