Diversey PassivationofStainlessSteel

8/10/2019 Diversey PassivationofStainlessSteel

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TECH TIPS Issue #15 April 2007

Passivation of Stainless Steel


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What is Chemical Passivation?Chemical passivation is a two step process.

The rst step is to remove any free iron or iron compound that is on the surface,otherwise this iron will create a localized site where corrosion can continue.

Acid is used to dissolve away the iron and its compounds. The surface itself is notaffected by this process.

The second step is to use an oxidizer to force the conversion of chromium metal on

the surface to the oxide form. This will create the uniform chromium oxide protec-tive layer.

How is it Done?Most Common Method Nitric AcidThe most commonly used chemicalmethod to passivate a stainless steelsurface is to apply nitric acid. Nitric is astrong mineral acid so it can quicklydissolve all iron compounds and othertrace metals that are on the surface.

Nitric acid is also a strong oxidizer so itcan generate the chromium oxide layer atthe same time.

Even though nitric acid is a strongchemical, high temperatures and extendedtimes are used to ensure the reaction iseffective and complete. The applicationcondition ranges are:

Time: 20 minutes to 2 hoursTemp: up to 70C (160F)Conc: 20 to 50% by volume

nitric acid

Note: Under these conditions nitric acid isvery aggressive on gaskets and it may benecessary to replace them after apassivation procedure.


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When to PassivateThere is no simple rule that says when apiece of equipment must be passivated.The need will vary according to how theequipment is being used and whether the

surface has been damaged. Some com-panies will choose to passivate processingequipment once per year as a scheduledmaintenance procedure. Other companieswill do it more frequently because theyare processing foods that are aggressiveon the stainless steel. Aggressive foodsare those that contain high chloride levelsand are acidic, for example salsa, tomato

juice, etc. Plants that use water that hasa naturally high chloride level may haveto passivate more frequently since thechloride will disrupt the protective layer.Pharmaceutical companies that use ul-tra pure water for injection are known topassivate 4 times per year because thehigh purity water itself is hard on the sur-face layer!Many times companies will passivatewhen they notice iron deposits formingon the stainless steel and it is not comingfrom the water. There are test kits avail-able from chemical supply rms that willtest for free surface iron. If a high level isfound, it could be time to passivate.

Other Methods1. Citric acid is also sometimes used for passivatingstainless steel. It is an acid that can effectively remove ironand its compounds from surfaces.

Time: variableTemp: ambient temp. to moderate heatingConc: 4 to 10% by weight

This method is being promoted because citric acid is safer touse than nitric acid, is biodegradable, produces fewer efuentconcerns and is also used as a food ingredient.

Citric acid does an excellent job of removing iron fromsurfaces, which is the rst step of traditional passivation. It isnot an oxidizer and so it cannot oxidize chromium which is thesecond step of classic passivation. It therefore cannot build upthe protective layer so this process depends on natural air oxi-dation. Citric acid is mostly used on small parts that will not beused in aggressive chemical or physical environments.

2. Dichromate is a very strong oxidizer that is sometimesadded to nitric acid solutions to improve the oxidation of chro-mium. Dichromate is toxic and is not permitted inindustrial efuent so food plants stay away from using it.

Recommended Products and Protocol

PascalThis is a high concentration nitric acidproduct suitable for passivation. Ensurecomplete rinsing after the passivation isdone.

Super DilacNot as high concentration of nitric acid butcan also be used in passivation.

Benet Chlorinated, alkaline product used to thor-oughly clean equipment, which has beenused in food production, prior to passiv-ation.

APC-NS or Solvent CleanerIf the surface has oils and greases on it,especially a new installation, it is recom-mended to remove them using a solvent-type cleaner at 2 4 ounces per gallon(15 30 ml/l) at 140F (60C) for30 minutes.


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JohnsonDiversey JohnsonDiverseyUnited States Canada3630 East Kemper Road 2401 Bristol CircleCincinnati, Ohio 45241 Oakville, Ontario L6H 6P1p:1-800-233-1000 p:1-800-233-1000f: 513-956-4841 f: 905-829-1218

Or visit our website at: www.johnsondiversey.com

JohnsonDiversey PASSIVATION PROTOCOLAREA:

EQUIPMENT: Silos, Tanks & Lines

PRODUCT CONC.Pascal 20%Benet (VC 62) 1%APC-NS 2 - 4%

Passivation Steps

SAFETY EQUIPMENT

GOGGLES/FACE SHIELD YESRUBBER BOOTS YESRUBBER APRON YESGLOVES YESHARD HAT NORESPIRATOR NO

TEST KITSAlkaline # 409879Acid # 409724

FREQUENCYYearly, or as required

CLEANED BY

SPECIAL PRECAUTIONSALWAYS WEAR PROPER PROTEC-TIVE CLOTHING. NEVER ADD WA-TER TO CHEMICAL ALWAYS ADDCHEMICAL TO WATER. REPORT

ALL ACCIDENTS IMMEDIATELY!

TOOLS AND EQUIPMENT

Developed byJohnsonDiversey

Training Department

CUSTOMER SERVICE

1-800-233-1000

CUSTOMER: PROTOCOL: Special ADDRESS: DATE:

Theory: Stainless steel is composed of iron (Fe); nickel (Ni);chromium (Cr) and several other minor components. Stainless steel isnot resistant to chemical or physical attack. The corrosion resistanceof stainless steel depends on the formation of a passive surface lmcomposed of nickel and chromium oxides (Cr 20 3 & Ni0).

Passivation involves removing free Fe or tramp Fe from the stainlesssteel surface. This is the purpose of Pascal. The nitric acid in Pascalenriches the stainless steel with Cr & Ni. The oxides (Cr 20 3 & Ni0) thenform and a passive surface now is in place.

Steps:1. If the equipment has been used, thoroughly wash the surface with

a chlorinated alkaline detergent such as Benet. If the surfaces arenew and machine oil may be present, utilize APC-NS. Keep in mindthe key to a good passivation is beginning with a SCRUPULOUSLYCLEAN SUR FACE.

2. Rinse the system with water.3. Prepare a 20% solution of Pascal (1 gallon of Pascal plus 4 gallons

of water). Heat the solution to 140F (60C) and circulate for 1 hourat temperature.

4. Drain the system and rinse with clean water. Check the pH of thewater to insure all the acid is removed

5. Allow the system to air-dry overnight.6. Sanitize all surfaces prior to production.

Notes:

The concentration of Pascal required to produce a passive surfaceon stainless steel may also be aggressive against gaskets if theyare old or of low quality. Therefore, it is best to perform thepassivation just prior to outtting equipment with new gaskets.

When to passivate? This depends on a number of factors, butgenerally passivation is preformed when:

-New equipment has been installed. -Repairs (i.e. welding) have occurred. -Aggressive products (high salt/low pH) are processed through

the equipment. -Equipment surfaces appear dull and dark.

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Diversey PassivationofStainlessSteel