The surface of Stainless Steel is actually an extremely thin but stable and passive Chromium rich oxide film, on which Stainless Steel relies for its excellent corrosion resistance. The surface finish on Stainless Steel should therefore be developed and maintained to ensure this vital property, and also for the secondary reason of the pleasing aesthetic appearance of Stainless Steel.
The Standard Mill Surface Finishes are laid down in Specifications (BS 1449, Part 4, and the Committee of Stainless Steel Producers, American Iron & Steel Institute).
The finishes are designated by a system of numbers, and these are broadly described hereunder relative to the finishing operations employed. It should be remembered that different grades of Stainless Steel can result in variation of visual appearance for the same finishing operation. The thickness can also have an effect, generally the thinner the material the smoother the surface finish.
The thicker gauge sizes of Stainless Steel are Hot Rolled. This is done at high temperatures and will always result in a scaled surface.
Stainless Steel Flat Product is supplied in the annealed, i.e. softened, condition. This is also a high temperature operation, and unless carried out in a very closely controlled inert atmosphere, will result in oxidation (scaling) of the surface. The scale is usually removed by a pickling process (the removal of the scale by use of suitable acids), and passivated by the use of Nitric Acid.
No 1 Finish
Plate is hot rolled, annealed, pickled and passivated. This results in a dull, slightly rough surface; quite suitable for industrial applications which generally involve the range of plate thicknesses. Grinding marks may be visible in isolated areas.
Some of the thinner thicknesses within the plate range are Cold Rolled; but Sheet, Coil and Strip gauges are produced by Cold Rolling, i.e. rolled without and heating of the material. Cold Rolling hardens the material, and the thinner sizes may have to be subjected to an intermediate anneal and pickle, or bright annealed, during the reduction of thickness to final gauge.
The starting material for Cold Rolling always has a No. 1 finish. Cold Rolled material is supplied with the following standard mill finishes.
No 2D Finish
A No. 1 Finish after being Cold Rolled, Annealed, Pickled and Passivated. This results in a uniform dull matt finish, superior to a No. 1 Finish.
Suitable for industrial applications, and eminently suitable for severe deep drawing as the dull surface (which may be polished after fabrication), retains the lubricant during the drawing operation.
No 2B Finish
A 2D Finish is given a subsequent light skin pass cold rolling operation between polished rolls.
This is the most common finish produced and called for on sheet material. It is brighter than 2D and is semi-reflective. It is commonly used for most deep drawing operations, and is more easily polished to the final finished required than is a 2D finish.
No. 3 Finish
This is a ground, unidirectional and uniform finish obtained with 80 - 100 grit abrasive.
It is a good intermediate or starting surface finish for use in such instances where the surface will require further polishing operations to a finer finish after subsequent fabrication or forming.
No. 4 Finish
This is a ground unidirectional finish obtained with 150 grit abrasive. It is not highly reflective, but is a good general purpose finish on components which will suffer from fairly rough handling in service (i.e. restaurant equipment). The final surface is blemish free with a high degree of image clarity, and is the true mirror finish.
Note: The finer polished finishes (No. 4, or greater) are generally only produced on one side of the sheet, the reverse side being either a 2B or No. 3 Finish.)
Passivation is the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel itself. In addition, passivation can be described as "the chemical treatment of stainless steel with a mild oxidant, such as a nitric or citric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film."
In layman’s terms, the passivation process removes "free iron" contamination left behind on the surface of the stainless steel from machining and fabricating. These contaminants are potential corrosion sites that result in premature corrosion and ultimately result in deterioration of the component if not removed. In addition, the passivation process facilitates the formation of a thin, transparent oxide film that protects the stainless steel from selective oxidation (corrosion).
Many machine shops, purchasing agents and engineers are somewhat in the dark when it comes to the relationship between corrosion resistant (stainless) steel and chemical passivation. Even among the finishing community, there is some disagreement about the theory behind the process of chemical passivation. Some believe it is effective because it is a cleaning process. Others credit the enhanced corrosion resistance properties to the thin, transparent oxide film resulting from chemical passivation. Regardless, the bottom line is that it works. Verification tests, including copper sulfate immersion, and accelerated corrosion tests, such as salt spray, high humidity and water immersion, undisputedly confirm the effectiveness of chemical passivation. Advanced material engineers in aerospace, electronics, medical and similar high-tech industries have used chemical passivation for years. The applications demand the maximum performance from components manufactured from corrosion-resistant steels, and they realize that passivation is one of the most effective methods of achieving these results.
The Electropolish process smoothens, polishes, deburrs, and cleans steel, stainless steel, copper alloys, and aluminum alloys in an electrolytic bath. The process selectively removes high points on metal surfaces, giving the surface a high luster.
The metal part is immersed in the Electropolish solution and subjected to direct current (DC). The metal part is made anodic (+) and a metal cathode (-) is used. The direct current the flows from the anode, which becomes polarized, allowing metal ions to diffuse through the film to the cathode, removing metal at a controlled rate. The amount of metal removed depends on the specific bath, temperature, current density, and the particular alloy being electropolished.
Conventional mechanical finishing systems tend to smear, bend, stress and even fracture the crystalline metal surface to achieve smoothness or luster. Electropolish offers the advantages of removing metal from the surface producing a unidirectional pattern that is both stress and occlusion free, microscopically smooth, and often highly reflective. Additionally, improved corrosion resistance and passivity are achieved on many ferrous and non ferrous alloys. The process micro and macro polishes the metal part. Micro-polishing accounts for the brightness and macro-polishing accounts for the smoothness of the part.
Deburring is accomplished quickly because of the higher current density on the burr, and because oxygen shields the valleys, enabling the constant exposure of the tip of the burr.
Because the part is bathed in oxygen, there is no hydrogen embrittlement to the part. In fact, Electropolishing is like a stress-relieve anneal. It will actually remove hydrogen from the surface.
Another benefit is that bacteria cannot successfully multiply on a surface devoid of hydrogen. This makes Electropolishing ideal for medical, pharmaceutical, semiconductor, and food-processing equipment and parts. The combination of no directional lines due to mechanical finishing, plus a surface relatively devoid of hydrogen, results in a hygienically clean surface where no bacteria or dirt can multiply or accumulate.
What are the Benefits of Electropolish?
· Stress Relief of the Surface
· Removes Oxides
· Passivation of Stainless Steel, Brass, and Copper
· Superior Corrosion Resistance
· Hygienically Clean Surfaces
· Decarbonization of Metals
· No Hydrogen Embrittlement
· No Direction Lines
· Low-resistance Welding Surface
· Reduces Friction
· Both Polishes and Deburrs Odd Shaped Parts
· Radiuses or Sharpens edges depending on rack position
· Reduces annealing steps
The most important corrosion consideration with stainless steels is pitting. Micro pitting is manifested as surface rust staining and is generally considered unacceptable for a material basically selected for its aesthetic appearance and corrosion resistance. Analysis and extrapolation of pit depth data gathered from atmospheric exposure test programs can be used as a durability guide. (This work was done by Corus (British Steel) Technical.) It must be borne in mind that staining from micro pitting may result in rejection of the steel on aesthetic grounds, long before pitting has perforated it.
Factors Affecting Durability
The durability of the stainless steel depends on:
· Steel Grade
· Surface Finish
Chloride ions are the most aggressive environmental hazard when assessing the pitting corrosion risk. Marine sites tend to be the most aggressive and so give the lowest projected durability.
Acid conditions, sometimes found in industrialized atmospheres, are also aggressive.
It is also important to consider the effects of local 'micro climates' that may influence how aggressive the environment is.
· Always attempt the mildest cleaning method first. Be patient – Repeat it a fair number of times before resorting to the more severe cleaning methods.
· Routine Cleaning: Stainless Steel’s best friends are quite simply soap, mild detergent or ammonia solutions in warm water, applied with a soft cloth or nylon sponge.
· Occasionally the use of the least coarse nylon scouring pad may be required. Rinse and dry with a soft cloth.
·Stainless steel articles are ideally suited for washing in a dishwasher. Only if cookware is heavily soiled is any prewashing required. (Note: Don’t wash in dishwashers which have galvanized (zinc plated) components. Indelible stains can result on the surface of stainless steel.)
· Such simple Routine Cleaning will easily remove normal soiling. Repeated application will often remove heavier soiling and stains will become less noticeable, and may completely disappear.
· Cleaning – Moderate Soiling, Light Staining: Apply the mildest household abrasive cleaner, or a paste made from fine chalk or soda bicarb, using a soft cloth or a fine nylon scouring pad. A soft bristle brush may also be used. Rub the surface as softly as possible, using long even strokes in the direction of the polished finish if this exists. Avoid using a circular rubbing action. Rinse well and wash as described under Routine Cleaning.
· Cleaning – Heavy Soiling Heavier Staining: Presoak in warm/hot detergent or ammonia solution. If this does not sufficiently soften burnt food or carbon deposits, household caustic cleaners will have to be used. Follow by cleaning as for Moderate Soiling, Light Staining. Repeat if necessary.
· If this does not suffice, final resort may have to be made to the use of both coarser abrasive cleaners and nylon scouring pads, but with the risk that the surface may become slightly affected. Follow by a thorough rinse and Routing Cleaning.
General Problems & Corrective Action
· Acids: Avoid contact. If accidental contact does occur, rinse immediately and soak in ammonia or soda bicarb solution. Follow by Routine Cleaning.
· Bleaches: Avoid contact with concentrated or undiluted bleach. If accidental contact does occur, rinse immediately and soak in ammonia or soda bicarb solution. Follow by Routine Cleaning.
· Carbon Deposits/Burnt Food: Treat as described for Heavy Soiling, Heavier Staining.
· Fats/Oils/Grease: Wipe off heavy deposits with a soft cloth or paper towel. Presoak in warm detergent or ammonia solution. Follow by Routine Cleaning.
· Finger Prints: Use Routine Cleaning. If necessary first treat the marks with a soft cloth or paper dampened with alcohol (methylated spirits), or an organic solvent (ether, benzene). Minimize the re-occurrence by applying a wax based household polish to the dry, cleaned surface.
· Films: A dull, cloudy film, or a “rainbow film” develops after drip drying. These are respectively due to too much detergent, or oil/grease in the washing-up water. Re-wash as for Routine Cleaning using fresh, clean water.
· Heat Discoloration/Tints: Ease of removal depends on the severity thereof. Repeated cleaning as for Moderate Soiling, Heavier Staining may prove successful. If not, use a 10% solution of Nitric Acid* together with a fairly course household abrasive cleaner applied as a paste by hard rubbing with a coarse nylon scouring pad. (Some alteration to the surface appearance with result from these severe cleaning operations) Acid treatment must be followed by rinsing in ammonia or soda bicarb solution, and Routine Cleaning.
· Labels: Peel off as much as possible. Soak well in warm water, rubbing periodically with a soft soapy cloth or sponge. If adhesive remains, dry and rub gently with alcohol or organic solvent.
· Leakage & Spills: Remove by thorough washing down as it occurs, or at short regular intervals.
· Oily Deposits in Coffee Pots/Urns: Use a thin paste of soda bicarb in hot water, and rub with a fine nylon scouring pad. Rinse and follow by Routine Cleaning.
· Rust Stains: Light, superficial brown staining can be removed by Routine Cleaning repeated regularly for a few days. Similarly, repeated cleaning as for Moderate Soiling, Light Staining will remove darker stains. Rust spots with a halo around them indicate that a fragment of ordinary steel has become embedded in the surface of the Stainless Steel. Dab and spot the stain (keeping moist for 20-30 minutes) with a 10% solution of Nitric Acid* on an ear bud. Repeat this treatment until no re-occurrence of the rust spot occurs. Severe rust stains are best removed by swabbing the stain, keeping it moist for 15-20 minutes with a 10% solution of Nitric Acid*, repeating if necessary. Very severe stains will require hard rubbing with a paste of fine household abrasive and 10% Nitric Acid*, using a fairly course nylon scouring pad. Acid treatments must be followed by rinsing in ammonia or soda bicarb solution, and Routine Cleaning.
· Sterilization: Add the sterilizing solution in the strength as laid down in the instructions. Avoid overdosing or the addition of concentrated solutions in one large dosage. Do not leave the sterilizing solution in the equipment longer than necessary, especially under stagnant conditions.
· Tannin Stains: Use a thin paste of sodium bicarbonate (washing soda) in hot water, and rub with a fine nylon scouring pad. Rinse and follow by Routine Cleaning.
· Nitric Acid (HNO3): Nitric acid is a “friendly” acid towards Stainless Steel and is the only mineral acid which should be used to clean stainless steel. Avoid contact of the acid with other metals, particularly the aluminum and copper bases on cookware. A 10% solution (1 part Nitric Acid added to 9 parts water) is usually used. It may be obtained from most chemists, who will make up the 10% solution. Concentrated Nitric Acid must be handled with care. Dilute 10% solutions are less dangerous, but it is advisable to wear rubber gloves and eye protection. If accidental skin contact occurs, wash well with lots of water. Mix and keep the solution in glass containers. Dilute extensively before flushing down the drain. Keep it out of reach of children.
1) Seal all container apertures.
2) Pressurize the container to 2.9 psig using a regenerative air source.
3) Coat all the seams and joints with a soap and water solution. (Soap solution recommended is 4 oz of soap to 3 gallons of water.
4) Pressure must be held for a period of time sufficiently long enough to assure detection of leaks, but no less than 5 minutes.
5) After completion of leak detection, safely depressurize the container.
Note: Care should be exercised when working with pressurized containers, as injuries may occur.
31A = Code Number Designating Type of IBC
Y = Packaging Group the IBC is Approved for (Group II & III)
MM/YY = Month/Year of Manufacture
USA = Country Authorizing the Marking
CMI = Name of Manufacturer
****kg = Maximum Stacking Test Load in kilograms
****kg = Maximum Gross Weight in kilograms
****L = Capacity in liters
****kg = Tare Mass in kilograms
MM/YY = Month & Year of Initial Leakproofness Test Date
MM/YY = Month & Year of Initial Inspection Test Date
**** = Material of Construction
***mm = Minimum Wall Thickness in millimeters
****kPa = Maximum Fill/Discharge Pressure in kilopascals
****** = Manufacturer’s Serial Number
****** = Custom Metalcraft Model Number
****gal = Capacity in US gallons
****lb = Maximum Gross Weight in pounds
****lb = Tare Mass in pounds
****** = Customer Serial Number
(1) “A” is the marking for all types and grades of steels.
(2) During transportation, vapor pressures within an IBC cannot exceed the limits imposed by CFR49 § 173.35. An IBC is not a pressure vessel.
(3) All IBCs are production tested at ≥ 20 kilopascals (kPa) (2.9 psig) air pressure.
(4) The UN code does not have to be on one line, but it does have to be sequential.
Online Chemical Compatibility Reference: http://www.coleparmer.com/Chemical-Resistance
U.S. Department of Transportation (DOT): www.dot.gov
Online Conversion Tool: http://www.onlineconversion.com/
Hazardous Cargo Bulletin: www.hazardouscargo.com
Question: Does DOT Specification 57 Tank (IBC) have to be converted to UN Specification IBCs for hazardous product use?
Answer: CFR 49 § 173.32 (d) Use of Specification 52, 53, 56 and 57 portable tanks. Continued use of an existing portable tank constructed to DOT Specification 52 or 53 is authorized only for a tank constructed before June 1, 1972. Continued use of an existing portable tank constructed to DOT Specification 56 or 57 is authorized only for a tank constructed before October 1, 1996.
Question: When should each IBC used for the transportation of a hazardous (regulated) material be successfully retested before further use?
Answer: Specification 57 portable tanks and UN31A portable tanks should be retested at least once every 2.5 years.
Question: How do I know if my portable tank has a current test date?
Answer: The date of the most recent periodic retest must be marked on the tank, on or near the certification plate.
Question: How do I document the retest date?
Answer: The owner of the tank or his authorized agent must retain a written record indicating the date and results of all required tests and the name/address of the tester, until the next retest has been satisfactorily completed and recorded.
Question: What if the test date expires while the tank is filled with hazardous product and in warehouse storage?
Answer: A portable tank for which the retest or re-inspection has become due may not be filled and transported until retest or re-inspection has been successfully completed. This does not apply to any tank filled prior to the test due date.
Question: What is the tare weight of an IBC?
Answer: Each IBC is weighed by the manufacturer and stamped on the certification plate on the IBC.
Question: How high can I stack my stainless steel IBCs?
Answer: Do not exceed the maximum stacking load listed on the certification plate on the IBC.