Stainless Steel Technical Presentation

7/27/2019 Stainless Steel Technical Presentation

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1. Stainless steel issues

2. Contamination

3. Introduction to the passive layer

4. Introduction to the process

5. Cleaning

6. Pickling

7. Passivating

8. Stainless Steel Testing Methods and Procedures

9. Stainless Steel ASTM Standards

10. Available Process Options From WestChem

Table of Contents:


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Contamination Rust Staining


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Rust staining from surface contact with ordinary carbon steel parts is often assumed to be corrosion of the stainless steel surface.

Iron Contamination can be easily avoided with careful handling, proper fabrication and controls, and can be removed with proper treatments

Staining can appear as a slight brown Bloom or rusty scour marks to pitting on items such as handrails.

Common sources of contamination include: Use of carbon steel tooling, processing and handling equipment (chains, bars, lifting

supports) without adequate clean down. Metal cutting and fabrication in mixed metal shops without appropriate segregation

or cleaning precautions( See Following Slides)

Mixed metal shops are likely to have some form of iron contamination present unless they take proper precautions and chemical procedures.

Contamination Rust Staining


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The result of the thickening of the naturally occurring oxide layer on the surface of the steel.

Colors from pale straw to dark blue and purple hues. As heat tints are formed, chromium is drawn to the surface of the

stainless steel. As chromium oxidizes more readily than the iron in the steel, the result is a layer at and below the surface with a lower chromium

level than in the bulk of the steel. The surface of the heat tinted area is now less corrosion resistant then the rest of the part.. It is good practice to remove all weld heat tint for the reason above. Removing the heat tint not only looks better aesthetically but also

restores the corrosion resistance properties of the stainless steel.

Welding Heat Tint


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Can be done by:1. Brush on pickling gels

2. Spray pickling3. Immersion Pickling

Pickling must be done AFTER the area has been cleaned and degreased (See Process options in slides below)

Contact times vary depending on temperature and severity of the heat tint.

Removing Weld Heat Tint


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Mixed metal shops should segregate stainless steel from their regular steel. Mixed shops should allocate specific stainless steel wire brushes (or grinding

pads) for use only on stainless steel. Never use one which has been used on regular steel as it will contaminate the surface of the stainless steel.

All saws and other fabrication equipment should be cleaned before running stainless steel. Saw blades should be marked and identified for use on only stainless steel.

Failure to follow the precautionary steps above will result is the transfer and contamination of iron particles to the surface of the stainless steel.

Iron removal with an acid solution is preferred over a single stage abrasive method such as wire brushing or nylon pad cleaning as it will enter all the tight areas where abrasive methods cannot.

Steel shot or grit if doing an abrasive blast. Also avoid using the same blasting media as used on any other material throughout the shop

Preventing Contamination


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The corrosion resistance of stainless steel is due to a passive chromium rich oxide film that forms naturally forms on the surface of the steel.

Stainless steels will naturally self passivate whenever a clean surface is exposed to an environment that can provide enough oxygen to form a chromium rich oxide layer.

Oxide layer will thicken over time Small areas deprived of oxygen can activate the steel. I.e mechanical

joints, tight corners, incomplete or poor welds. Can result in localized pitting and reduced corrosion resistance.

Introduction: The Passive Layer


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The Passive Layer


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The full process is:1. Clean Removes oils, grease and contaminates.

2. Rinse Removes cleaning chemicals and contaminates.3. Pickle Removes small portion of surface metal and passive layer.4. Rinse Rinses off pickling agent and the passive layer starts forming upon drying.5. Passivate Removes any remaining free iron and accelerates the formation of the passive

layer

6. Rinse Rinses off passivator and any left over contaminates.

*Note: It is encouraged that the final rinse of any process be RO or Distilled Water*

Introduction To The Process


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Pickling alone cannot be relied on to remove heavier oils, grease, or other inorganic contaminants that can also stop the passivation layer from

forming properly.

If parts are contaminated with oils or grease, then cleaning prior to pickling should be carried out.

Both Acid and Alkaline cleaners are acceptable for degreasing. Your

WestChem Representative can recommend the right one for your specific application If parts are to be pickled after cleaning, then it is preferred that the

surface is 100% water break free (See Next Slide)

Having a water break free surface assures that the pickling acid reacts evenly with the substrate, creating a uniform reaction and appearance.

Step #1: Cleaning


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Ensuring a 100% Water Break Free surface allows for an even and consistent pickling action. (See Next Slide)

Water Break Free Surfaces


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Patchy, uneven pickled surfaces can result if surfaces are not clean before pickling (Acid Treatment).

Cleaning Continued


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Removing cleaner from step #1. Prevents cross contamination of chemicals Prepares the substrate for optimal reaction of the next chemical

step (pickling). High pressure impingement removes loose oils, soils, dirt and

dust.

If poor water conditions exist, RO or DI Water is recommended.

Step #2: Rinsing


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Types:1. Immersion tank2. Spray Pickling3. Thick paste/gel applied with acid resistant brush on all welds and other critical areas

Immersion is best as it pickles all surfaces and gets 100% coverage of the parts.

Pickling times vary with temperature of the substrate. If substrate is being brought in from a cold temperature, it is recommended to warm the substrate to room temperature or rinse the part with very hot water.

Must be careful when handling these chemicals as they are strong. Take proper precautions and use proper PPE

It is very important that all pickling residues are rinsed away completely to achieve a fully corrosion resistant and stain free surface.

*NOTE: Citric based pickling methods VS. Nitric/Sulphuric based methods:

Nitric pickling is a more widely accepted method, but Citric is more environmental/user friendly and may or may not give as good of pickling results as Nitric.

Pickling Methods


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In most cases, it is advisable to use a pickling paste/gel on all welds as they are potential corrosion sites. This is required when not using a gel

pickling acid

on

the

entire

part.

The removal of a thin layer of metal from the surface of the stainless steel.

The use of Nitric and Hydrofluoric acid is common when pickling. Removes weld smoke and heat tint on welded areas Removes free iron and passivation layer. After the pickling product is

rinsed off, the chromium at the surface will oxidize with the air and start developing a new passive layer on the stainless steel.

This passivation layer restores the corrosion resistant properties of the stainless steel.

Although passivation is automatic, it can be assisted and accelerated with other chemical treatments (coming up in passivation section).

Step #3: Pickling


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Removing pickling chemistry from step #3. Prevents cross contamination of chemicals Prepares the substrate for optimal reaction of the next chemical

step (passivating). High pressure impingement removes loose oils, soils, dirt and

dust.

If poor water conditions exist, RO or DI Water is recommended.

Step #4: Rinsing


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After pickling, flash clouds or brown staining may be evident. This step helps remove them, along with free iron and water staining.

Process occurs naturally on the surface of stainless steel, but sometimes it is necessary to assist the process with an oxidizing acid treatment

Steel from a reputable mill will come Passive. When you start fabricating (welding, grinding, machining) you will have to passivate the

surface to

ensure

it

is

corrosion

resistant.

Quality and thickness of the passivation layer is quickly developed during this treatment

Passivation occurs when the chromium oxidizes with exposure to oxygen

in the air to form the corrosion resistant passivation layer, this process is accelerated with a passivation agent.

Step #5: Passivation


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Removing passivation chemistry from step #5. This is the final thing that makes contact with your part before it

becomes a final product. High pressure impingement removes loose oils, soils, dirt and

dust.

RO or DI Water is recommended for the final rinse.

Step #6: Rinsing


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Most common method to test that the stainless steel has been passivated is the Copper Sulphate test. Other common methods for testing are salt spray, humidity and water immersion.

Copper Sulphate Test Procedure

1. Mix 4 Grams Copper Sulphate Pentahydrate into 250 mls of water that contains 1 ml of sulphuric acid.

2. Apply Copper Sulphate solution via spray to a specific area that is to be tested. Smaller parts can also be immersed in a Copper Sulphate.

3. Let Copper Sulphate dwell on the parts for 6 minutes

4. Rinse with water

5. If any areas where the Copper Sulphate was applied turn pink, the test is a fail. This indicates there is free iron left on the surface and can lead to corrosion in the field.

Salt Spray, Humidity Testing and Water Immersion

1. Minimum of 2 hour salt spray with NO rusting/staining is a pass.

2. Minimum of 24 hour humidity testing with NO rusting/staining is a pass.

3. Immersed in Distilled water for 1 hour and air dried for 1 hour. Repeat 12 times. NO

rusting/staining is a pass.

Stainless Steel Testing Methods and

Procedures


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There are two common ASTM Standards for the treatment and passivation of stainless steel. They are as follows:

ASTM A380 Spec Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems

ASTM A967 Chemical Passivation Treatments for Stainless Steel Parts

WestChem Technologies has the most current of these ASTM specs on file.

They can be emailed if requested.

Stainless Steel ASTM Standards


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Depending on your application, fabrication process, and customer specifications, WestChem can tailor a process that suits your needs.

High Performance

Process:

WestKleen SS 200 or WestKleen SS 240 > Rinse > WestGel SS 500 > Rinse > WestInhib 26 > Rinse (RO)

It may be beneficial, depending on weld quality, to pre brush welds with WestGel SS 500.

Enviro Performance Process: WestKleen SS 200 or WestKleen SS 240 > Rinse > WestKleen SS 232 or Corrosol

52959 + WestGel SS 500 on welds > Rinse > WestInhib 26 > Rinse (RO)

Mid Performance

Process:

WestKleen SS 200 or WestKleen SS 240 > Rinse > WestKleen SS 232 or Corrosol52959 + WestGel SS 500 on all welds > Rinse (RO)

Stainless Steel Products Offered From

WestChem Technologies


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Minimum Performance Process: WestKleen SS 232 or WestGel SS 500 > Rinse > WestInhib 26 > Rinse

(preferably RO

Water)

* If your material contains oils and grease, this process may not provide proper cleaning and consistent pickling and passivating.*

**One step process may not remove all free iron contamination.**

WestChem carries a combination of ready to use products as well as mixed options depending on application, equipment, and plant

requirements.It is always advised to provide samples of the stainless steel to

WestChem Technologies for matrix testing which includes panel preparation, salt spray, and copper sulphate testing.

Stainless Steel Products Offered From

WestChem Technologies

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Stainless Steel Technical Presentation