passivation of ss closed system

7/24/2019 Passivation of SS closed system

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Passivation of Stainless Steel

Passivation describes the treating of a metal with a mild oxidant (such as nitric acid) to removesurface iron or iron compounds by dissolution. This action forms a protective passive film on thsurface of the metal. The trace iron left behind from machining and fabrication can provide site

for corrosion if left untreated.

The process is performed by first cleaning the surface with solvents or an alkaline solution toremove organic or metallic residues. Next, the parts are placed into passivation solutions. Thechoice for this solution is usually nitric acid. The process is controlled by three key variables:time, temperature, and concentration.

Typical Range of Key VariablesTime: 20-120 minutesTemperature: 60 - 160 0FConcentration: 20 - 50% Nitric Acid by volume

The type of stainless steel alloy being processed typically determines the key variables. Forexample, 316 stainless steel would have different conditions than 317L stainless steel.

After the passivation bath, a sodium dichromate bath is often used to promote the formation of chromic oxide film. After this final treatment, the sample is placed into a copper sulfate solution

Any remaining iron will show up as pink spots on the sheet. This would be consideredunacceptable. Other testing methods can include a two hour salt spray or a 24 hour highhumidity test.

What is passivation?

According to ASTM A380, passivation is "the removal of exogenous iron or iron compounds frothe surface of stainless steel by means of a chemical dissolution, most typically by a treatmewith an acid solution that will remove the surface contamination, but will not significantly affethe stainless steel itself." In addition, it also describes passivation as "the chemical treatment stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancinthe spontaneous formation of the protective passive film."

In lay terms, the passivation process removes "free iron" contamination left behind on thsurface of the stainless steel from machining and fabricating. These contaminants are potenticorrosion sites that result in premature corrosion and ultimately result in deterioration of thcomponent if not removed. In addition, the passivation process facilitates the formation of a thitransparent oxide film that protects the stainless steel from selective oxidation (corrosion). Swhat is passivation? Is it cleaning? Is it a protective coating? It is a combination of both.

How is passivation performed?

The process typically begins with a thorough cleaning cycle. It removes oils, greases, formincompounds, lubricants, coolants, cutting fluids and other undesirable organic and metallresidue left behind because of fabrication and machining processes.


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General degreasing and cleaning can be accomplished many ways, including vapor degreasinsolvent cleaning and alkaline soaking.

After removing organic and metallic residues, the parts are placed into the appropriapassivation solution. Although there are many variations of passivating solutions, th

overwhelming choice is still the nitric-acid-based solutions.

Recently, there has been substantial research performed to develop alternative processes ansolutions that are more environmentally friendly, yet equally effective. Although alternativsolutions containing citric acid and other types of proprietary chemistry are available, they havnot been as widely accepted commercially as nitric-acid-based solutions.

The three major variables that must be considered and controlled for the passivation processelection are time, temperature and concentration. Typical immersion times are between 20 mand two hours. Typical bath temperatures range between room temperature and 160F. Nitracid concentration in the 20 to 50% by volume range is generally specified. Many specificationinclude the use of sodium dichromate in the passivation solution or as a post passivation rinse taid in the formation of a chromic oxide film. Careful solution control, including water purity, ppof metallic impurities and chemical maintenance, are crucial for success.

The type of stainless steel determines the most effective passivation process. Bath selectio(time, temperature and concentration) is a function of the type of alloy processed. A thorougknowledge of the material types and passivation processes is paramount to achieving thdesired results. Conversely, improper bath and process selection and/or process control wproduce unacceptable results. In extreme cases, this can lead to catastrophic failure, includinextreme pitting, etching and/or total dissolution of the entire component.

Equipment and precautions

Trained, experienced technicians familiar with the potential hazards associated with the sciencshould only perform passivation. Safety practices must be fully understood when handlinpassivation chemicals. Special boots, gloves, aprons and other safety equipment must be useTanks, heaters and ventilation, as well as baskets and racks must be appropriately engineereto perform the process. Iron or steel parts or equipment must never be introduced to thprocess, or the results can be devastating. Furthermore, in order to comply with EPrequirements, the necessary water and air permits and treatment capabilities must be in placThe days of mom-and-pop shops performing passivation in a stone crock in the back of the shoare gone.

Specifications and verification testing

There are a few generally accepted industry specifications available for reference whechoosing a passivation process. They offer time, temperature and concentration information ansubsequent testing requirements to validate the effectiveness of the process. Many largcorporations have developed internal specifications to control their unique requiremenregarding passivation and verification testing. Regardless of the situation, it is usually prudent reference a proven procedure when requesting passivation.


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By referencing a specification, you do not have to reinvent the wheel. By taking advantage of thexperiences of others, both successes and failures, you can eliminate much of the guesswothat would otherwise accompany a new process.

Although recently canceled, the most commonly referenced industry specifications regardin

passivation are Federal Specification QQ-P-35C, which is now superseded by ASTM A-967 anASTM A-380. All are well-written, well-defined documents that provide guidance on the entiprocess, from manufacturing to final testing requirements. If you are not sure what you neethey can be referenced in full or selectively. The testing requirements can be used or waivedepending on the individual situation.

One of the most commonly specified verification tests is the copper sulfate test. Passivated parare immersed in a copper sulfate solution for six min, rinsed and visually examined. Any copp(pink) color indicates the presence of free iron and the test is considered unacceptable.

Other validation tests include a two-hr salt spray or 24-hr high-humidity test. Placing passivateparts in a highly controlled chamber that creates an accelerated corrosive environment performthese tests. After subjecting the test pieces to the corrosive atmosphere for the prescribeexposure periods, the parts are removed and evaluated. Although results can be somewhsubjective, ASTM B-117 is an excellent reference in determining acceptability. It is important note that each of the test methods mentioned have different advantages and limitations. Carshould be taken to select the appropriate test methods based on alloy type and end-usenvironment.

Machining and heat treating techniques

Perhaps the most overlooked variable in the entire passivation equation is the negative impaof poor machining and heat treating practices. All too often, cross contamination introduceduring manufacturing and/or thermal processes leads to unacceptable test results. The followinpractices will reduce cross contamination during manufacturing and increase the chances successful passivation and tests results.

Never use grinding wheels, sanding materials or wire brushes made of iron, iron oxide, steelor other undesirable materials that may cause contamination of the stainless-steel surface.

The use of carbide or other non-metallic tooling is recommended. Grinding wheels, sanding wheels and wire brushes that have been previously used on other

metals should not be used on stainless steel. Use only clean, unused abrasives such as glass beads or iron-free silica or alumina sand for

abrasive blasting. Never use steel shot, grit or abrasives that have been used to blast othermaterials.

Thorough cleaning prior to any thermal processing is critical. Stress relieving, annealing,drawing or other hot-forming processes can actually draw surface contaminants deeper intothe substrate, making them almost impossible to remove during passivation.

Care should be taken during all thermal processes to avoid the formation of oxides. Passivatiois not designed to remove discoloration and will not penetrate heavy oxide layers.


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In extreme situations, additional pickling and descaling operations are required prior passivation to remove the discoloration. Controlled atmosphere ovens are highly recommendefor all thermal processes to reduce airborne contamination and prevent oxides from developing

So how do you get the performance you have paid for from high-dollar stainless steel alloys?

boils down to a basic understanding that the passivation process is both an art and a sciencand that machining, fabricating and heat treating practices can substantially affect the corrosioresistance of the component. Passivation will enhance the corrosion resistance of stainlessteels, but to realize the maximum performance from these high-tech alloys, all parties involvewith manufacturing must understand their responsibility in maintaining the integrity of thmaterial throughout the process.

PITTING

The mechanism of pitting is essentially the same as crevice corrosion. The difference is thwhereas in crevice corrosion the phenomenon is derived from the fact that a crevice alreadexists, pitting needs to be initiated - take for example cases of pitting in the tank tops in thcentre of a steel plate, nowhere near a weld seam or a crevice.Pitting is avoidable. Like crevice corrosion, only one of the factors leading to it has to bremoved to stop the reaction.

When a clean tank is inspected and pitting is observed, then it is a case of locking the stabdoor after the horse has bolted. We are seeing the result of a previous corrosion cell. When wmeasure the passivity of the steel, even inside a corrosion pit, it almost invariably shows passive area. Passivity is the unlikelihood of a particular area to corrode. Inside a pit undinspection conditions the electrolyte has been removed, as far as is observable. Inside thpitting the steel has access to oxygen and the passive chromium oxide layer has been restoreThis is no reason, however, to ignore the pitting. The chromium concentration on the surface othe pit will be less than that of the surface surrounding it. The pit can trap seawater again in thfuture and reactivate a corrosion cell.

The pitting has to be removed mechanically and filled by the correct metal. The resulting repamust be devoid of crevices. In other words, the repair must have the same characteristics as thsurrounding steel. This is achieved by careful welding followed by the minimum removal of nematerial by mechanical means to achieve a smooth finish. This is followed by firstly activatinthe steel in the area by pickling to remove unwanted oxides and contamination, followed bpassivation to restore the passivating chromium oxide layer. Weld spatter must be similarremoved from the surrounding area and the area similarly pickled and passivated.

Pitting is often associated with areas of uniform corrosion. For example, an area of steel abo1m2 is rougher than the area surrounding it. Inside this area are examples of even worscorrosion in the form of pitting, either shallow, open pitting or deep pinpoint pitting.

It is necessary to look at the causes of pitting to avoid it in the future.

Pitting is caused by starving a surface of the steel of oxygen relative to the surroundinsurfaces, followed by exposing both to an electrolyte.


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To starve a steel surface of oxygen you need to contaminate it.

Some examples of contamination are :

Free iron. If iron from tools or boots imbeds itself into the stainless steel surface, then th

surface will rust and the rust will form a porous layer on the stainless steel surface. The stesurface is deprived of oxygen but an electrolyte can penetrate through to the steel surface. Whave created an 'artificial crevice'.

Organic deposits, Mud or silt, Remains of impure cargoes such as wet phosphoric acid. This usually the cause of the uniform deterioration of the surface texture of the steel, i.e. it becomerough or abrasive.

Following the discharging of wet phosphoric acid, a thorough tank cleaning is required. This needed to remove the tightly adhering sediments present in the acid. If this is not done, the largareas of the tank surface are deprived of oxygen. Subsequent immersion in an electrolyte wset up a gigantic corrosion cell between the 'clean' areas and the 'dirty' areas. These sedimentas far as corrosion is concerned, have the same properties as rust - they are adherent anporous to electrolytes.

The result is, that although through repeated use of the tank, eventually it is clean, a uniforcorrosion has occurred during the time it was dirty and exposed to electrolytes. The dirty areahaving sacrificed themselves as anodes to the clean cathodes.

Similarly, in depressions in the tank top, where sediments and electrolytes can colleundisturbed, limited uniform and associated pitting can and does occur.

Depending upon the nature of the cargoes to be carried in the future (electrolyte or nonelectrolyte; i.e. aqueous or organic (solvents etc.) then it is advisable during a period such adry-docking to restore the surfaces of the affected tanks to a uniform quality by mechanicmeans such as pitting repair or even polishing of rough areas, followed by complete picking anpassivation. In that way, the surface quality of the tank will once again be as intended whenew.

Passivation meter for stainless steel surfacesThe passivation meter sets up a temporary corrosion cell. The anode is the stainless stesurface and the cathode is the reference electrode. The electrolyte is a suitable solution for thapplication and is applied drop wise to a piece of absorbent paper, which is then placed on thsteel surface. The tip of the reference electrode is then placed on top of the soaked papethereby making a steel / electrolyte / electrode connection. The electrode is connected to a mmeter and another contact extending from the meter to the steel completes the corrosion cell.

The mV meter contains a microchip, which performs the function of reducing the referencelectrode's absolute potential to zero. The mV reading seen on the LCD gives an indication the tendency that the steel will readily corrode.


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The manufacturer has set the limit that any reading above 60 mV indicates a passive surface, oin other words, an intact chromium oxide layer on the steel surface.

A reading of less than 30mV shows that the surface is still in an active state.

A reading of 30mV


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Passivation Procedure for 300-Series Stainless Steels

Step 1

Prepare a 50% (v/v) nitric acid solution by pouring an equal volume of concentrated nitric ac

into an equal volume of water in a container that will not be attacked by the solution. (Nitric acis available in 50% strength. Phosphoric acid may be used as a substitute.) Best results will bobtained if the temperature of the passivating solution is maintained at 130 F (54 C). Usextreme caution when using nitri c acid; it is a very dangerous chemical.

Step 2

Apply the passivating solution to the complete surface. Cover all areas several times. Keep thsurface wet with the acid solution for about 1 h. With a small vessel (15 gal or less), it is easiesimply to fill the vessel with the passivating solution and let it soak. When dealing with largevessels, passivation via a spray-ball system is very effective.

Step 3

Rinse the vessel thoroughly with clean, warm water to remove all traces of acid. Wipe with clean cloth (wet or dry). If no discoloration appears on the cloth, the surface is conditioned anready to use.

Passivation is not a permanent treatment. Stainless steel surfaces can become recontaminateand corroded if proper care is not taken.

Scope:

This pickling and passivation procedure is designed to remove weld discoloration, heat treatingscale, and iron contamination resulting from the fabrication process, from austenitic stainlesssteel. Reference: ASTM, A380-88 Wyandotte Technical Service Bulletin I-85

Process requirements:

Parts to be pickled shall be thoroughly cleaned of all grease, oil, or dirt by others, prior tdelivering parts to Orbit Industries, Inc. Use of suitable alkaline cleaner such as WyandotNuvat is recommended.

Pickling: (Removes weld discoloration, heat treating scale and iron contamination) There are twmethods of pickling: Spraying method. Fill or Immersion method. Contact time to be 4 to hours. Solution is Nitric Acid with additive.

Inspect the pieces and verify that discoloration, scale, and foreign material have been removed

Passivation: (Removes iron oxide loosened by pickling and renders the metal surfaccompletely clean)


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Rinsing removes all material loosened by the pickling procedure.

The rinsing procedure must follow directly after pickling so that the solution does not dry. Rinswith clean water, preferably a hot high pressure wash, and a chloride content no greater than 5mg/liter.

Contact with air after rinsing procedure will establish a passive film on the surface of thestainless steel (Passivation)

Inspection: Assemblies shall be inspected in accordance with applicable customer requirementand Job Quality Control Procedures.

passivation of ss closed system

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