You have ensured that the parts are machined to specification.Now, make sure you’ve taken steps to protect these parts in the conditions your customers expect.#basic
Passivation remains a critical step in maximizing the basic corrosion resistance of stainless machined parts and assemblies.It can make the difference between satisfactory performance and premature failure.Improperly executed, passivation can actually cause corrosion.
Passivation is a post-fabrication method that maximizes the inherent corrosion resistance of the stainless steel alloys that produce the workpiece.It is not a descaling treatment, nor is it a paint coating.
There is no general consensus on the precise mechanism of how passivation works.But it is certain that there is a protective oxide film on the surface of passivated stainless steel.This invisible film is thought to be extremely thin, less than 0.0000001 inch thick, about 1/100,000th the thickness of a human hair!
A clean, newly machined, polished or pickled stainless steel part will automatically acquire this oxide film due to its exposure to atmospheric oxygen.Under ideal conditions, this protective oxide layer completely covers all surfaces of the part.
In practice, however, contaminants such as shop dirt or iron particles from cutting tools can transfer to the surface of stainless steel parts during machining.If not removed, these foreign bodies can reduce the effectiveness of the original protective film.
During machining, trace amounts of free iron can wear off the tool and transfer to the surface of the stainless steel workpiece.In some cases, a thin layer of rust may appear on the part.This is actually corrosion of the steel by the tool, not of the base metal.Occasionally, crevices of embedded steel particles from cutting tools or their corrosion products can cause erosion of the part itself.
Likewise, small particles of ferrous shop dirt may adhere to the surface of the part.Although metal may appear shiny in the machined state, after exposure to air, invisible particles of free iron can cause surface rusting.
Exposed sulfides can also be a problem.They come from adding sulfur to stainless steel to improve machinability.Sulfides increase the alloy’s ability to form chips during machining, which can be completely sloughed off the cutting tool.Unless parts are properly passivated, sulfides can become a starting point for surface corrosion on manufactured products.
In both cases, passivation is required to maximize the natural corrosion resistance of the stainless steel.It removes surface contaminants, such as ferrous shop dirt particles and iron particles in cutting tools, that can form rust or become a starting point for corrosion.Passivation also removes sulfides exposed on the surface of free-cutting stainless steel alloys.
A two-step procedure provides the best corrosion resistance: 1. Cleaning, a basic but sometimes overlooked procedure; 2. Acid bath or passivation treatment.
Cleaning should always be a priority.Surfaces must be thoroughly cleaned of grease, coolant or other shop debris for optimum corrosion resistance.Machining debris or other shop dirt can be carefully wiped from the part.Commercial degreasers or cleaners can be used to remove process oils or coolants.Foreign matter such as thermal oxides may have to be removed by methods such as grinding or pickling.
Sometimes a machine operator may skip basic cleaning, mistakenly thinking that cleaning and passivation will happen simultaneously by simply dipping a grease-laden part in an acid bath.That won’t happen.Conversely, contaminated grease reacts with acid to form air bubbles.These bubbles collect on the workpiece surface and interfere with passivation.
To make matters worse, contamination of passivation solutions, which sometimes contain high concentrations of chlorides, can cause “flashing.”Unlike obtaining the desired oxide film with a glossy, clean, corrosion-resistant surface, flash etching can result in a heavily etched or darkened surface—surface deterioration that passivation is designed to optimize.
Parts made from martensitic stainless steel [magnetic, moderately resistant to corrosion, yield strength up to about 280 ksi (1930 MPa)] are hardened at elevated temperatures and then tempered to ensure the desired hardness and mechanical properties.Precipitation hardenable alloys, which have better strength and corrosion resistance than martensitic alloys, can be solution treated, partially machined, aged at lower temperatures, and then finished.
In this case, the part must be thoroughly cleaned with a degreaser or cleaner to remove any traces of cutting fluid prior to heat treatment.Otherwise, the cutting fluid remaining on the part can cause excessive oxidation.This condition can cause undersized parts to dent after the scale has been removed by acid or abrasive methods.If cutting fluid is allowed to remain on bright hardened parts, such as in a vacuum furnace or protective atmosphere, surface carburization can occur, resulting in loss of corrosion resistance.
After thorough cleaning, the stainless steel parts can be immersed in a passivating acid bath.Any of three methods can be used – nitric acid passivation, nitric acid with sodium dichromate passivation, and citric acid passivation.Which method to use depends on the grade of stainless steel and the specified acceptance criteria.
More corrosion-resistant chrome-nickel grades can be passivated in a 20% (v/v) nitric acid bath (Figure 1).As shown in the table, less resistant stainless steel can be passivated by adding sodium dichromate to a nitric acid bath, making the solution more oxidizing and able to form a passive film on the metal surface.Another option to replace nitric acid with sodium chromate is to increase the concentration of nitric acid to 50% by volume.Both the addition of sodium dichromate and the higher concentration of nitric acid reduce the chance of undesired flash.
The procedure for passivating free-machining stainless steels (also shown in Figure 1) is somewhat different from that for non-free-machining stainless steel grades.This is because during passivation in a typical nitric acid bath, some or all of the sulfur-containing machinable grade sulfides are removed, creating microscopic discontinuities in the surface of the machined part.
Even a generally effective water rinse can leave residual acid in these discontinuities after passivation.This acid will then attack the surface of the part unless it is neutralized or removed.
To effectively passivate easily machinable stainless steel, Carpenter has developed the AAA (Alkali-Acid-Alkali) process, which neutralizes residual acid.This passivation method can be completed in less than 2 hours.Here is the step-by-step process:
After degreasing, soak the parts in a 5% sodium hydroxide solution at 160°F to 180°F (71°C to 82°C) for 30 minutes.Then rinse the parts thoroughly in water.Next, immerse the part for 30 minutes in a 20% (v/v) nitric acid solution containing 3 oz/gal (22 g/l) sodium dichromate at 120°F to 140°F (49°C) to 60°C). After removing the part from the bath, rinse it with water and then immerse it in the sodium hydroxide solution for another 30 minutes.Rinse the part again with water and dry, completing the AAA method.
Citric acid passivation is increasingly popular with manufacturers who wish to avoid the use of mineral acids or solutions containing sodium dichromate, as well as the disposal issues and greater safety concerns associated with their use.Citric acid is considered environmentally friendly in every way.
While citric acid passivation offers attractive environmental advantages, shops that have had success with inorganic acid passivation and have no safety concerns may want to stay the course.If these users have a clean shop, well-maintained and clean equipment, coolant free of ferrous shop fouling, and a process that produces good results, there may be no real need for changes.
Passivation in a citric acid bath has been found to be useful for a large range of stainless steels, including several individual stainless steel grades, as shown in Figure 2.For convenience, the traditional nitric acid passivation method in Figure 1 is included.Note that older nitric acid formulations are expressed in volume percent, while newer citric acid concentrations are expressed in weight percent.It is important to note that when implementing these procedures, careful balancing of soak time, bath temperature and concentration is critical to avoid the “flashing” described earlier.
Passivation treatments vary according to the chromium content and machining characteristics of each grade.Note the columns referencing either Process 1 or Process 2.As shown in Figure 3, Process 1 involves fewer steps than Process 2.
Laboratory tests have shown that the citric acid passivation process is more prone to “flashing” than the nitric acid process.Factors contributing to this attack include too high bath temperature, too long soak time, and bath contamination.Citric acid products containing corrosion inhibitors and other additives such as wetting agents are commercially available and are reported to reduce susceptibility to “flash corrosion”.
The final choice of passivation method will depend on the acceptance criteria imposed by the customer.See ASTM A967 for details.It can be accessed at www.astm.org.
Tests are often performed to evaluate the surface of passivated parts.The question to answer is, “Does passivation remove free iron and optimize corrosion resistance of free-cutting grades?”
It is important that the test method matches the grade being assessed.Tests that are too strict will fail perfectly good materials, while tests that are too loose will pass unsatisfactory parts.
400 series precipitation hardening and free-machining stainless steels are best evaluated in a cabinet capable of maintaining 100% humidity (wet samples) for 24 hours at 95°F (35°C).The cross section is often the most critical surface, especially for free-cutting grades.One reason for this is that the sulfide is elongated in the machine direction, intersecting this surface.
Critical surfaces should be placed upwards, but at 15 to 20 degrees from vertical to allow for moisture loss.Properly passivated material will hardly rust, although it may show some slight staining.
Austenitic stainless steel grades can also be evaluated by humidity testing.When so tested, water droplets should be present on the surface of the sample, indicating free iron by the presence of any rust.
The procedures for passivating commonly used free-cutting and non-free-cutting stainless steels in citric or nitric acid solutions require different processes.Figure 3 below provides details on process selection.
(a) Adjust pH with sodium hydroxide.(b) See Figure 3 (c) Na2Cr2O7 represents 3 oz/gallon (22 g/l) sodium dichromate in 20% nitric acid.An alternative to this mixture is 50% nitric acid without sodium dichromate
A faster method is to use the solution in ASTM A380, “Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems.”The test consists of wiping the part with a copper sulfate/sulfuric acid solution, keeping it wet for 6 minutes and observing for copper plating.As an alternative, the part can be immersed in the solution for 6 minutes.If the iron dissolves, copper plating occurs.This test should not be used on the surfaces of food processing parts.Also, it should not be used for 400 series martensitic or low chromium ferritic steels as false positive results may occur.
Historically, the 5% salt spray test at 95°F (35°C) has also been used to evaluate passivated samples.This test is too stringent for some grades and is generally not required to confirm that passivation is effective.
Avoid using excess chlorides, which can cause harmful flash attacks.If possible, use only high-quality water with less than 50 parts per million (ppm) chloride.Tap water is usually sufficient and can tolerate up to several hundred ppm chloride in some cases.
It is important to replace the bath regularly to avoid loss of passivation potential that can lead to flashover and damaged parts.The bath should be maintained at the proper temperature, as runaway temperatures may cause localized corrosion.
It is important to maintain a very specific solution change schedule during high production runs to minimize the potential for contamination.A control sample was used to test the effectiveness of the bath.If the sample is attacked, it is time to replace the bath.
Please specify that certain machines make stainless steel only; use the same preferred coolant to cut stainless steel, excluding all other metals.
DO rack parts are treated individually to avoid metal-to-metal contact.This is especially important for free machining stainless steel, as free-flowing passivation and flushing solutions are required to diffuse corrosion products in sulfides and avoid the formation of acid pockets.
Do not passivate carburized or nitrided stainless steel parts.The corrosion resistance of parts so treated may be reduced to the point where they would be attacked in the passivation bath.
Do not use ferrous tools in a workshop environment that is not particularly clean.Steel grit can be avoided by using carbide or ceramic tools.
Don’t forget that corrosion can occur in the passivation bath if the part is not heat treated properly.High carbon, high chromium martensitic grades must be hardened for corrosion resistance.
Passivation is usually carried out after subsequent tempering using temperatures that maintain corrosion resistance.
Do not ignore the nitric acid concentration in the passivation bath.Periodic checks should be made using the simple titration procedure provided by Carpenter.Do not passivate more than one stainless steel at a time.This prevents costly confusion and avoids galvanic reactions.
About the authors: Terry A. DeBold is a stainless steel alloy research and development specialist and James W. Martin is a bar metallurgist at Carpenter Technology Corp. (Reading, PA).
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