What is stainless steel passivation?
In a general context, the term “passivation” means something becoming chemically passive, where passive is the opposite of active. Active surfaces react readily while passive surfaces are resistant to reactions, including corrosion reactions.
Most metals, aluminum and titanium for example, are generally self-passivating. Exposed surface atoms readily react with the oxygen in air to form a stable layer of passive metal oxide. If steel tools are used on such metals, trace amounts of iron can be left on the surface, and the iron will rust just as any iron would. A passivation treatment for such metals removes that iron from the surface using an acid solution.
Chromium and sometimes also nickel is added to steel to create stainless steel and impart the ability to form stable metal oxides to the alloy. However, the alloy content is still mostly iron, which commonly forms an oxide that deteriorates the surface. This iron that is inherent to the metal, as well as contaminant iron from external sources, can lead to surface rusting. Once again, the passivation treatment refers to removing that surface iron with an acid solution. In this case, the other components of the alloy are left behind as a surface layer over the underlying steel, and upon exposure to air they react with oxygen to form the passive metal oxide layer that protects the rest of the steel from corrosion.
This passive oxide layer can be damaged through mechanical processes, once again exposing iron that is subject to rusting. This is why passivation is the final step in any manufacturing process of stainless steel parts. Welding creates a heat affected zone in which the alloy structure is altered, also typically needing passivation to restore the corrosion resistance. Chemical damage to the passive layer can be caused by exposure to chlorides. Bleach, salt, swimming pools, marine environments, and hydrochloric/muriatic acid are especially harsh on stainless steel.
You may also hear the term passivation used for other processes on other metals that create a stable, unreactive surface. One of the common ones is the chromate conversion coating process used over zinc or a zinc-coated substrate, usually iron or steel. Whereas with stainless steel, the chromium is already present in the alloy. The black iron oxide process is another common one, used to create a layer of an alternate form of iron oxide that is more stable than red rust iron oxide
How do I know which CitriSurf product to use?
Please contact us for chemical passivation support in selecting a CitriSurf product that is best for your application. Primary considerations are the grades of stainless steel being used and the size of the parts.
The standards say I need a 4 percent citric acid bath, how does that relate to CitriSurf?
The 4-10 weight percent given in the standards refers to dry, pure citric acid. Most CitriSurf products are provided as a liquid concentrate. Our product literature gives dilution instructions in terms of volume percent and volume ratio, and the maximum dilution given for each product corresponds to a resulting 4 weight percent citric acid in the working solution. In order to create a correct passivation solution, please do not confuse CitriSurf concentrate with dry citric acid or weight percent with volume percent.
How do I know if my parts are successfully passivated?
Some people associate the light surface etch that can occur from nitric acid with passivation. In contrast, the appearance of the parts is not typically changed by passivation with CitriSurf. Accepted passivation test methods are given in ASTM A967 and other industry standards. The copper sulfate test is the most common quick and easy method for grades with high chromium content. Copper Sulfate test kits are available from Stellar Solutions.
Do I need to pre-clean my parts?
It is generally not necessary if there is only light oils or dirt on the parts. CitriSurf products are excellent cleaners and can handle light grease and oils. However, a KleerKleen product can be used as a precleaner if needed. Also, stainless steel alloys high in sulfur or carbon usually require a pretreatment in an alkaline bath such as KleerKleen 4002 in order to remove these elements and prevent appearance issues. It is best to discuss this with us.
Should I heat my CitriSurf bath?
The passivation process requires less time at higher temperatures, so we recommend heating when available, but room temperature is usually sufficient as well.
Can I use CitriSurf products with ultrasonics?
Yes, in fact we encourage the use of ultrasonic tanks when they are available.
Do I have to use deionized (DI) water with CitriSurf?
No. You may use any type of clean water you choose for dilution and rinsing. Certain industries have purity concerns and restrict themselves to only using DI or another type of purified water. For this reason CitriSurf is manufactured using DI water. However, using average tap water has no negative effect on the passivation process itself. Using purified water for the final rinse bath can help avoid waterspotting issues.
How long does a CitriSurf bath last?
CitriSurf will not lose effectiveness due to age, but the acid does deplete through the processing of parts. This depends on the quantity, geometry, and condition of the parts being passivated and is difficult to predict. Use bath monitoring procedures recommended by Stellar Solutions to monitor the acid concentration and maintain the effectiveness of your bath. Contact us for assistance with this.
Are CitriSurf products safe to use on food equipment and other sensitive surfaces? Do they leave a coating or residue?
CitriSurf may be used for these applications. When properly rinsed, no CitriSurf remains on the surface. The protective passive layer is formed from the metal itself. For our products that are most used in the food, medical, and pharmaceutical industries, we have registration in the NSF Nonfood Compounds program to confirm this.
Will CitriSurf products affect the finish of the stainless steel?
Unlike nitric acid based passivation, a properly selected CitriSurf product will not etch stainless steel and will not change the surface finish.
Can CitriSurf products be used on metals other than stainless steel?
CitriSurf may be used to remove surface iron from nearly any metal, including titanium, aluminum, hastalloy, inconel, cobalt chrome, etc. It is also works as a good brightener for copper, brass, and bronze.
Are CitriSurf products safe for rubber, plastic, and brazings?
Almost all plastics and rubbers are compatible with citric acid and CitriSurf. The metals commonly used for brazing are also compatible. Please check with the manufacturer of your material for complete information. PVC pipe is not recommended for long term use with acids such as CitriSurf.
Are there any materials CitriSurf is not compatible with?
CitriSurf will cause damage to the surfaces of concrete, carbon steel, zinc, and galvanized steel. Avoid exposure of these materials to CitriSurf and rinse immediately if there is an inadvertent spill onto these materials. CitriSurf can be used to clean carbon steel surfaces if care is taken to avoid overexposure, we recommend extra dilution and room temperature operation. PVC pipe and fittings made from copper, brass, or bronze are not recommended for long term use with CitriSurf.
Can CitriSurf products be used on laser marked parts?
Laser marks on stainless steel are very susceptible to corrosion if they are not passivated, due to the heat stirring up fresh iron to the surface. One of the best properties of CitriSurf is its ability to get good corrosion resistance of laser etch marks. Adjusting the laser parameters may help also. It is best to discuss this with us.
Note that for nonferrous alloys such as titanium, laser marking is not a process that can create iron contamination and a need to passivate.
Will CitriSurf products clean off heat scale or weld discoloration?
It varies depending on the severity of the scale/discoloration, but in general citric acid does not attack the surface enough to remove these. Our Weld Wizard line of products is better able to safely remove heat and weld discoloration from the surface of your stainless steel.
Will CitriSurf pickle and passivate?
Many people tend to conflate the two types of treatment, so here’s a clarification of the terminology: Pickling is the removal of discoloration and scale by an acid treatment. Its purpose it to make the surface appearance more uniform. Passivation is the removal of surface iron by an acid treatment, its purpose is to help in preventing rust corrosion. Citric acid based passivation, including CitriSurf, does not pickle. It’s not strong enough to etch or eat into the metal surface. Nitric acid passivation does produce some pickling (and nitric pickling produces some passivation), which is why a lot of people still think the two things are a single process. If someone says they want pickling and passivation, ask if they are looking for just corrosion resistance or if scale/discoloration removal is needed. They possibly only want passivation and don’t know it.
What are electropolishing and electropickling? Are they the same as passivation?
Electropolishing uses a continuous electric current, applied through carefully arranged electrodes in a specially mixed acid bath, to remove metal atoms from the microscopic “peaks” on the surface and create a very smooth surface finish. Electropolishing is often referred to as a passivation method since smooth surfaces are more corrosion resistant than rough surfaces. However, this is not the same process as nitric/citric acid passivation, as the electropolishing removes all metal atoms to smooth the surface, rather than preferential iron removal. In fact, a citric acid passivation treatment after electropolishing will produce the benefits of both methods combined.
Electropickling is also often billed as a passivation method, but its goal is to use the electric current just long enough to remove the discoloration or scale. It is not normally used to produce a smoother surface, and neither does it preferentially remove iron, so it is doubtful that it is beneficial for corrosion resistance.
Will CitriSurf products remove rust?
Several CitriSurf products are especially formulated for faster rust removal as well as passivation. Also, the Rust Rescue products help increase the protection in highly corrosive environments and on pitted surfaces.
My stainless steel item is a railing, sculpture, or otherwise too large and unwieldy for immersion in a bath. What product can I use?
CitriSurf 2210 and 77 are especially made for such applications, and can be used at your location or on site. For items that have been exposed to the environment and have started developing corrosion, CitriSurf 2310 or 77 Plus should be used.
Can outdoors on-site passivation be performed during the winter months?
The time needed for passivation is related to the temperature. Room temperature or higher is preferred, but outdoor passivation in cold seasons may be done if the exposure time is extended accordingly.
Will a higher concentration of CitriSurf yield better passivation?
Yes, but higher temperatures and longer exposure times have a greater effect on improving the passivation.
Are CitriSurf products ASTM/SAE/RoHS/NSF/etc. compliant?
Yes. Please see the Passivation Standards page.
Do we need a ventilation system to use CitriSurf products in our plant?
No. CitriSurf releases no acid fumes. When heated, only water vapor is emitted. Standard HVAC is typically sufficient for ventilation.
How are CitriSurf products shipped? Can they ship by overnight air?
For most of our products, the 5 gallon containers and other small sizes ship by UPS ground unless otherwise requested. Overnight shipping can be used, though it can of course be very expensive.
Are CitriSurf products sold at any retail stores?
Several of our ready-to-use products are available to the consumer market within the United States from The Rust Store.
CitriSurf products are also available from our distributors around the world. If no distributor is near you, please contact us directly.
Does stainless steel passivate on its own?
If you clean the surface well, then the chromium on the surface will certainly passivate into chromium oxide. However, at best that will only give 20% coverage since there’s typically only that much or less chromium in the alloy. A proper acid passivation bath (citric or nitric) increases the surface coverage of the chromium by removing the surface iron, resulting in improved corrosion resistance.
Is it true that nitric acid passivation oxidizes but citric acid passivation doesn't?
The idea that nitric acid forms the passive chromium oxide layer because it is an oxidizer while citric acid does not is a common misconception among the old guard who are misinformed on the mechanisms of passivation and basic chemistry. Due to the successes with citric acid we now understand that the chromium oxide layer is formed when the surface is exposed to air, and not while the surface is submerged in the acid.
Among chemists, the term oxidation can mean two different things. The chemical reaction of an oxygen atom joining to another atom is the most familiar definition. This reaction involves the other atom giving some of its electrons to the oxygen atom, and this electron exchange is also called oxidation, even when the atom taking the electrons is not oxygen, and even when a chemical bond is not formed between the two atoms. This distinction is where the confusion lies.
All acids are oxidizers, and when the passivation acid removes iron from the metal surface, that iron is being oxidized, i.e. some of its electrons are taken away. It is true that nitrate, the other part of nitric acid, can also be an oxidizer (taking electrons), while citrate, the other part of citric acid, is not. However this does not play a role in the formation of the oxide layer because neither material donates an oxygen atom to join with the surface chromium to form the metal oxide. Oxygen can only combine with the surface chromium if an oxygen donor is present, such as oxygen gas in air, or an oxygen donor chemical such as peroxide that could be (but typically isn’t) added to the passivation acid bath.
What's that word again? Passify? Passification? Passivization?
Though “passive” and “pacify” sound similar, the word pacify comes from latin meaning “to make peace”, while the word passive comes from latin meaning “to suffer”. A metal surface that is passive (non-reactive) is able to suffer through exposure to the environment, so to speak. We say that we passivate parts during the process of passivation in order to achieve that passive surface.