Citric Acid Passivation of Stainless Steel
Using Chem-Crest® ProPass and Crest Ultrasonics precision cleaning.
Chem-Crest ProPass is a stellar solution for passivation and oxidation removal used on various grades of stainless steel. It can also remove multiple levels of rust and oxidation from different types of steel and deoxidize various metals, including brass and aluminum.
Our phosphate-free citric acid passivation concentrate and deoxidizer cleaner is designed to remove surface contamination, increase corrosion resistance, and remove discoloration, tarnishes, scales, and oxides. It is an alternative for nitric acid passivation and passes citric acid passivation evaluations such as ASTM A967. Please contact your local Crest Ultrasonics representative for further information.
Chem-Crest ProPass is compatible with stainless steel, brass, bronze, copper, zinc, and aluminum. It is safe to use on glass, ceramics, and plastics. For ferrous (non-stainless steel) surfaces, you must protect them immediately after ProPass treatment with an appropriate rust inhibitor.
Why Chem-Crest ProPass?
Price. Delivery. Effectiveness. It is that simple.
Chem-Crest ProPass is priced lower than our primary competitor, and we offer fast shipping on orders in gallon, case, pail, drum, or tote sizes.
Our clients for ultrasonic stainless steel passivation represent many industry sectors, including aerospace, medical, dental, and industrial.
Please contact your local Crest Ultrasonics representative for application-specific questions and learn how Chem-Crest ProPass can further optimize your cleaning and passivation needs.
Why does stainless steel require passivation?
“Passivation” involves procedures that remove metallic or “free” iron from the surface of stainless steel materials. These free iron ions on a stainless steel surface can cause corrosion or rust spots.
Contaminants can easily compromise stainless steel and expose opportunities for corrosion. Here are some common examples:
- Tools used to machine or repair components leave microscopic contaminants from the cutting surfaces of bits or blades
- Handling or maintenance introduces surface damage
- Dirt or shop dust settles on the surface
- High amounts of sulfides used to ease machining contaminating stainless alloys
Such contamination can damage stainless steel by disrupting the naturally occurring thin layer of chromium oxide that protects it from atmospheric oxygen and, therefore, corrosion. This delicate protective layer is approximately 1/100,000th the thickness of a human hair.
The chromium oxide layer forms virtually instantly when manufacturing exposes the stainless steel to air. So any material or contamination that becomes embedded in or spread over the surface will likely compromise the layer and introduce corrosion directly onto the metal.
Passivation is a post-fabrication method that returns the inherent corrosion resistance of the stainless steel part as initially produced. It is not a scale removal treatment, nor is it like a coat of paint. It is a cleaning methodology that allows the original chromium oxide layer to re-form naturally.
The benefits of using citric acid passivation
Both a nitric acid passivation solution and a citric acid passivation solution can reestablish the protective chromium oxide layer. Manufacturers historically used nitric acid passivation before more recent scientific studies proved that citric acid passivation provides a safer, more effective method of passivation with many advantages and attributes, such as:
Citrus fruits – oranges, lemons, limes, etc. – are the main ingredients used in citric acid passivation. All are easy to grow and ship.
Federal law allows the storage of citric acids as a non-hazardous material. The nitrogen oxide vapors (NOx) emitted from nitric acid contribute to smog and acid rain and damage the zone layer. While nitric acid presents an environmental hazard, none of this is true, of course, with a citric acid passivation solution. In fact, it requires minimal waste treatment.
Because citric acid is non-hazardous, you avoid hazardous waste removal or regulatory oversight costs. You also avoid paying costly overhead for special handling equipment and safety devices. NOx vapors can also corrode other equipment and structures in the building where it is used, requiring repairs and replacements.
Additionally, nitric acid passivation requires a much higher solution concentration than the required citric acid concentration.
Faster removal of contaminants
In certain instances, passivation baths using citric acid can increase production up to five times the pace of nitric acid passivation baths.
Safer to use
Homes and businesses have long used citric acid as an effective ingredient in cleaners and disinfectants. It has a glowing reputation as a safe chemistry.
Begin with a Crest Ultrasonics bath
The first citric acid passivation process step you must take is to precision clean the parts, eliminating all surface contaminants — including those on every twist and turn and blind corner of the piece.
Ultrasonic passivation offers superior rust inhibition over dip-tanks or manual scrubbing. Ultrasonic cavitation occurring during an immersion bath allows micro-scrubbing of the outer layer of the material, reaching all exterior and interior surfaces of complex parts. Used alongside passivation chemistry, you can quickly restore the surface’s passive film and remove any possibility of oxidation forming.
A worker places all items into a metal basket. He or she then lowers into a Crest Ultrasonics bath, where cavitation (think “imploding miniature bubbles”) lifts all oils and debris. The basket then rinses and lowers into a citric acid bath of ProPass Citric Acid Solution, and then rinsed again and dried.
A typical ProPass Citric Passivation clean line will use the following recipe, which you can use with our Digital Modular Series (DMS) ultrasonic cleaning equipment, among others.
Crest equipment typically used
Using Crest Ultrasonics’ automated ultrasonic passivation equipment combines cleaning, rinsing, passivation, rinsing, and drying within one working system — freeing your operators to complete other tasks and reducing your overall cleaning costs.
Machine operators might skip even basic cleaning, assuming that both cleaning and passivating occur at once by immersing a grease-laden part in an acid bath. No, that isn’t how it works. Instead, the acid will cause the contaminating grease to form gas bubbles, which will collect on the part’s surface and interfere with passivation. Lack of pre-washing can also cause a “flash attack,” wherein the part’s surface becomes heavily etched or darkened, essentially ruining the piece. In the case of martensitic-grade stainless steel, parts might even need to go through a vapor degreaser to remove cutting fluids that could cause excessive oxidation and lower corrosion resistance.