Laser systems remove corrosion, grease, residue and existing coatings from metal surfaces quickly and without the mess. Photo Credits: Laser Photonics

Most manufacturers understand the value of pretreating metal surfaces of parts to remove corrosion, grease, residue, old coatings, or to roughen the surface of metals prior to coating. By ensuring the items are cleaned down to bare metal, manufacturers can avoid costly warranty issues that result when coatings peel, flake, bubble or otherwise fail prematurely.

The traditional techniques used for this purpose — such as sandblasting, dry ice blasting, and chemical stripping — are messy and require expensive consumables, as well as substantial time for preparation and cleanup. These methods are also drawing scrutiny from regulators such as the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) since they can pose risks to the environment and applicators.

A more effective alternative is using industrial-grade, precision laser-based systems that can remove paint, contaminants, rust and residues with a high-energy laser beam that leaves the substrate unaffected. Preparation and cleanup time are minimal, and the low-maintenance equipment can last decades.

According to Vincent Galiardi, owner of Galiardi Laser Clean, a surface cleaning operator based in St. Charles County, Missouri, many people are surprised to learn that clean technology lasers are one of the most cost-effective, efficient and safe methods of industrial surface preparation.

The company has implemented laser systems made by Laser Photonics, a provider of industrial-grade CleanTech laser systems for cleaning and surface conditioning. The American-made systems function either as mobile standalone units or can be integrated into production lines.

Given its effectiveness in pretreating metal surfaces, these laser systems are increasingly being used in manufacturing facilities. The systems can be integrated into automated processing lines or technicians can use mobile handheld units. With significant advantages in safety and efficiency, laser cleaning is poised to disrupt the surface pretreatment market across more sectors.

Conventional cleaning limitations

Many applications in manufacturing require pretreatment of metal surfaces prior to coating. To improve coating adhesion, residue, oil or grease must be removed before coating. In some cases, a manufacturer may seek to further enhance coating adhesion by roughening the surface.

When defective metal parts are produced, instead of discarding the product,

CleanTech laser systems are available in portable and stationary models ranging from 50 watts to 3,000 watts with chamber sizes from 3 ft. x 3 ft. to 6 ft. x 12 ft. The systems can also be installed in manufacturing lines in cabinets or operated by a robotic arm. The units have a touchscreen that enables simple customization of several laser profiles to meet the needs of any project. 

manufacturers can strip the paint and re-coat the component. To refurbish existing metal parts or recoat industrial infrastructure, removing the previous coating along with any corrosion is usually required to facilitate the new coating’s adhesion to the surface.

To pretreat metal surfaces, sandblasting, dry ice blasting or chemical stripping are traditionally used as industrial cleaning processes.

Sandblasting. Abrasive sandblasting involves forcefully projecting a stream of abrasive particles onto a surface, usually with compressed air or steam. The silica sand used in abrasive blasting typically fractures into fine particles and becomes airborne, which can cause serious or fatal respiratory disease.

When workers inhale crystalline silica, the lung tissue reacts by developing fibrotic nodules and scarring around the trapped silica particles, causing a fibrotic lung condition called silicosis. Estimates indicate that more than one million U.S. workers are at risk of developing silicosis and that more than 100,000 of these workers are employed as sandblasters.

In addition, particles are generated during abrasive blasting that further contribute to respiratory problems and other harmful health effects.

“Industry has needed a cleaner, safer surface pre-treatment solution for a very long time,” Galiardi says. “Sandblasting is inherently unsafe for operators. The silica glass used in sandblasting is toxic. An operator must wear a full HEPA suit when sandblasting to avoid breathing in particulates.”

Sandblasting is also time-consuming to clean up since the sand essentially scatters everywhere, even though it is usually considered a “fast” cleaning method.

“When we treat a surface with lasers, any fumes or dislodged particulate is extracted into a HEPA filter and the job is done.”

Dry ice blasting. With dry ice blasting, dry ice pellets are used as the abrasive. The challenge is that dry ice blasting is often not abrasive enough to sufficiently remove paint or corrosion from the surface of metals. Since dry ice is an expensive consumable, the costs can escalate when cleaning metal surfaces in higher volumes.

Chemical stripping. With chemical stripping, harsh, even toxic chemicals are used to strip metal-based objects of paint, rust and other contaminants to bare metal. However, for operators, exposure to corrosive acids and noxious chemical fumes is inherently dangerous. The process can also be time-consuming to prepare the proper chemical bath, achieve the required level of cleaning, and dispose of the waste. In addition, disposing of toxic chemicals is costly and closely regulated by agencies like OSHA and the EPA.

A clean, safe and fast approach

Laser-based systems have significant advantages over these traditional methods, including ease of use in which an operator simply points and clicks a high-energy laser beam at the surface. The substrate is not affected by the laser, and the systems do not create any mess or byproducts. The approach is eco-friendly, energy-efficient and completes the job in half the time of traditional methods when preparation and cleanup are considered.

“In our experience, laser cleaning is as fast at removing rust or old coatings as other methods, but without the same amount of cleanup,” Galiardi explains. “When we treat a surface with lasers, any fumes or dislodged particulate is extracted into a HEPA filter and the job is done. There is no media to replenish or clean up.”

CleanTech laser systems are available in portable and stationary models ranging from 50 watts to 3,000 watts (a 4,000-watt version is in development) with chamber sizes from 3 ft. x 3 ft. to 6 ft. x 12 ft. The systems can also be installed in manufacturing lines in cabinets or operated by a robotic arm. The units have a touchscreen that enable simple customization of several laser profiles to meet the needs of any project. 

Corrosion and oil removal

Galiardi says laser pretreatment of metal surfaces can be used to streamline various manufacturing processes. Corrosion, for example, can begin to accumulate within a very short time on new parts, depending on the material and environmental conditions and should be removed prior to coating.

For one major auto manufacturer, Galiardi Laser Clean was asked to remove rust from conveying system components used to transport cars through the manufacturing process. The components were corroded due to being left outside during a six-month delay in the project. When it was time to install the items, the provider wanted to first treat the surfaces and return the components to a “like new” appearance.

With laser cleaning, prep and cleanup time can positively impact project cost.

In another example, Galiardi was asked to remove rust from over 400 transmissions in a couple of days. The laser systems are particularly effective when reaching into tight spaces that are hard to reach by hand. By masking the area to protect vulnerable parts, the laser can be applied without affecting the rest of the assembled product.

“No other parts [of the transmission] had to be removed and nothing had to be cleaned afterwards,” he says.

Galiardi’s company also used the laser system to remove cleaning oils from truck chassis. “We used the laser to remove the oil right before painting so it was a bare metal object going with nothing on it that would affect the coating,” he says.

Industrial plants that need to recoat existing metal structures also need to remove rust before painting. According to Galiardi, he removed corrosion from a large storage tank using the CleanTech laser system in about half the time of the alternative being considered, an abrasive disc grinder.

“Disc grinders basically just chip off [the rust] and it becomes airborne and makes a mess. Grinders can also be dangerous because sparks or debris can shoot off the wheel or catch an article of clothing,” he says.

With laser cleaning, prep and cleanup time can positively impact project cost. When the improved operator safety, equipment longevity and lower maintenance of laser systems are also considered, the clean laser technology has a much higher ROI, according to Galiardi. The longevity of low-maintenance laser systems further adds to their value, increasing ROI, and making replacement unnecessary for decades.

“CleanTech laser systems can last for 50,000 to 100,000 hours,” he adds. “That’s many decades working eight-hour days. After purchase, there’s virtually no maintenance necessary.”

About the Author

Photo Credit: Stephen Armstrong

Stephen Armstrong

Stephen Armstrong is a freelance writer who has researched and written about industrial technologies, healthcare, automotive and international trade for the past 15 years.

Atotech, an MKS brand, presents a high-performance coating process that reduces energy consumption and CO₂ footprint.

Zintek 400, a zinc flake coating that can provide greater than 2,000 hours neutral salt spray protection on large components, can be used with XLink 800, a cross-linking technology that enables the coating to dry/harden at ambient temperature. 

The pretreatment process includes a low-temperature, long life cleaner, near neutral pH descale, zirconium pretreatment and ambient temperature adhesion promoter. 

This latest coating process in the Atotech CP3.0 program expands the scope of applications that can use zinc flake coating technology.

Q: We’ve been having trouble getting good coverage in our immersion iron phosphate process prior to powder, resulting in several reworks and off-spec parts. This issue arose about the same time our supplier switched to a stearate-based lube, so I believe we need to reassess our cleaning process, but not sure where to start?

A. Iron phosphate chemistries and other pretreatment methods are designed to provide enhanced physical and performance properties to the finished part or surface. These applications are employed in various industries — from automotive and aerospace to construction, firearms and others — to improve the durability and reliability of the product in the field. While a few factors influence the efficacy of the phosphate coating, adequate cleaning and surface preparation are paramount to ensuring that the coating is uniformly applied and effectively adhered.

The primary objective of any surface preparation is to ensure that the substrate is entirely free of organic and inorganic contaminates and otherwise address any characteristics hindering the development or quality of the impending phosphate application. Organic contaminants are characterized as carbon-based molecules; this includes natural and synthetic oils, pre-existing rust inhibitors, machining and metalworking fluids, waxes, lubricants and more. Inorganic contaminants are another form of surface imperfection and a common inhibitor to effective coatings; these include mill and heat-treat scale, base metal surface oxidation (corrosion/rust), and smut. The targeted removal of all present contaminants is a key aspect of a successful surface preparation and phosphate coating.

Cleaning and surface prep can be achieved mechanically, chemically, or through a combination of both. Mechanical methods can include blasting and wire brushing, and while these are effective techniques, they are often not practical or cost-effective at scale for many applications. Chemical applications use solvent or aqueous products to process and prepare parts at scale through batch or continuous cycles.

Aqueous cleaners are vast and varied in formulations and functions, often designed with a focus on specific applications or purposes. Typically, the first level of classification or segregation would be based on the cleaner’s primary builder(s); consequently, this translates to its relative acidity/alkalinity as well:

• Acid-based cleaners are formulated using mineral and organic acids, resulting in pH levels <6. These cleaners are commonly, but not exclusively, used in non-ferrous substrate applications and are highly effective in de-rusting, deoxidizing, and descaling various inorganic contaminants.
• Neutral to mildly alkaline cleaners can be made with a host of builder chemistries, including phosphates, amines and silicates. These cleaners can range in pH from roughly 6.5–10.5 and are generally multi-metal safe.
• Caustic cleaners utilize sodium and/or potassium hydroxide and have pH levels >11; these are often very effective in cleaning ferrous substrates and are commonly used prior to phosphate coatings. Under the right operating conditions, these products can also effectively de-rust surface oxidation.

In addition to these builders, aqueous cleaners possess a range of specialty components to address various contaminants. Organic soils generally have low-to-no solubility in water, so aqueous cleaning applications employ surfactants packages to increase their solubility and remove them from the substrate. Caustic cleaners can also facilitate the chemical reaction of saponification, whereby larger organic molecules (soils) are cleaved into fatty acid salts or soaps. Additionally, cleaners can use several other agents to offer properties such as chelation, sequestration, mild corrosion protection, brightening, and more.

Beyond the chemical makeup of the cleaner, three vital operating parameters play a critical role in the cleaner’s performance: time, temperature, and concentration. These parameters are direct variables to balance the application’s needs and limitations. Phosphate baths typically use caustic-based cleaners as the substrates are customarily ferrous and caustic cleaners tend to be the most robust. Immersion time is typically in the range of 5-10 minutes but can extend as necessary. Temperature is also quite variable, with operating ranges from 150°F up to 200°F; temperatures at the top of that range can be required for more tenacious materials such as stearate lubes and polymeric compounds. Lastly, concentration also influences the efficiency and activity — immersion cleaners typically run at 5-10% v/v but can be run at up to 20% v/v for applications where corrosion or extensive contamination is present. Modifying any of these parameters can improve the cleaning performance and, in turn, improve the phosphate’s crystal formation, coverage, and adhesion properties.

Other aspects worth assessment include the set-up and maintenance of the cleaning operation. Immersion cleaners can be either emulsifying or oil-splitting. Emulsifying cleaners are ideal for barrel-processed parts and are functional until the surfactant capacity is saturated. Conversely, oil-splitting cleaners are preferred for racked parts and in applications where considerable organic contaminants are being removed. The fixturing with respect to the part’s geometry can also impact the cleaner’s performance as recessed areas and concavities need adequate solution contact and flushing action. Active filtration of the cleaning solution ensures that the bath is not becoming oversaturated with soils. Lastly, bath maintenance is undoubtedly something that should not be overlooked. Daily checks for temperature, concentration titrations, and visual inspection of the cleaning solution ensure that work is processed consistently according to your application’s specifications. Judicious care for the daily operations of the cleaner baths translates to mitigated periodic maintenance as bath quality is kept in spec, leading to few bath dumps and tank cleanouts. These, among others, are all influencing factors to be aware of in evaluating and troubleshooting a cleaner issue.

Considering the totality of the cleaning application and understanding the variables in effect is crucial to evaluating and troubleshooting any cleaning issue. Each application is unique, from the equipment being used, to the part’s substrate and geometry, to the contaminants needing to be addressed. Often, a minor adjustment in the operating parameters or fixturing can address the issue; other times, with more substantial changes, you may need to consider alternative chemistries better suited to your application. Consult your chemical supplier for troubleshooting guidance and product input whenever in doubt.

About the Author

 

Photo Credit: Products Finishing

Connor Callais

Connor Callais is an applications specialist for Hubbard-Hall. Visit hubbardhall.com.

Rodger Talbert has more than 30 years of experience in the powder coating industry.

Q: We have begun to take on some automotive components that fit under the dash, inside the car interior and under the hood. The appearance standards are not too hard for us to achieve but we are a little concerned about the corrosion requirements. Some of them are just 500 hours of neutral salt spray, but some are 1,000 hours and that may be more challenging. We are able to pass the salt spray test but we have had some field failures (rust). The rust occurs on edges usually but it can also occur in corners and sometimes on the flat areas of the part. We use a five-stage iron phosphate wash process. Some of the parts require a hybrid powder and a few have some UV requirements that we use a TGIC powder coating material to meet. Any suggestions on how we can better ensure corrosion resistance would be appreciated. 

A: While salt spray has been used for many years by the automotive industry as a standard the salt spray results and field performance are not closely connected. Salt spray can be useful to help compare different pretreatment methods and coatings but it does not tell us much about the corrosion resistance of a part over time in the field.

Using salt spray to qualify a process is ok if you can verify it regularly to confirm the reliability of the process. If you pass a certain level of salt spray one day and do not pass on another day, there may be something that is not working properly and needs to be brought back into a safe range. In testing, remember to always use a control panel from a test facility to ensure the problem is not related to your steel material. A polished ACT or Q panel should always be part of your testing. Run at least three panels or parts to establish reliability.

Iron phosphate does not add much to the corrosion resistance. I would suggest you consider a zirconium oxide or zinc phosphate to improve performance. The zirconium oxide will improve corrosion resistance and does not have the environmental challenges of a zinc phosphate. Before you do the chemical treatment, be sure you have clean weld areas and round the sharp edges if you can. The chemical pretreatment will not remove weld smut and you need better edge coverage based on your description. You can grind or blast those inorganic soils and you will get better performance. You should also consider a powder primer coat. The primer will add a lot of corrosion protection. It will also help to round those edges and give you more coating at critical locations. A double coat of powder is always superior for edge coverage and corrosion protection.

For reliable testing of corrosion, you should consider a cyclic corrosion test. Putting a part through wet/dry cycles will provide a better correlation to exposure in the real world. Different specifications exist or can be created as to the specific cycles to use for a given industrial application. If you are doing automotive work, your customer can provide you with the right specification. SAE J156315 is a good source for guidance on cyclic testing. Do the testing routinely and use the results for the development of reliable control ranges for your pretreatment process and application.

DSP Bresle Chloride Field Test Kit. Photo Credit: Paul N. Gardner Co.

Paul N. Gardner USA (Gardco), worldwide distributors, producers and designers of quality physical and inspection instruments for the paint, coatings and related industries is offering the DSP Bresle Chloride Test Kit from TQC. The test kit includes all necessary equipment for assessing the level of soluble salts on blast-cleaned surfaces prior to coating, the company says.

Inside the Bresle Kit is a conductivity gauge used for the assessment of soluble salt ions as chlorides, sulphates and nitrates. The Bresle Kit complies with ISO 8502-6 and ISO 8502-9 standards to assess the level of soluble salts using a Bresle patch or Bresle sampler, distilled water and a conductivity gauge. The conductivity is mainly directly proportional to the concentration of dissolved chloride ions in the solution.

According to the company, the distinct direct sampling procedure (DSP) featured in the kit promotes high speed and accuracy — up to 60 times more accurate than other test kits available.

The Bresle Chloride Test Kit is also suitable to determine the contamination of blast media in use. This test, described in the ISO 11127-6 and ISO 11127-7 standards, helps reduce the risk that dissolved salts in the recycled abrasive media or water will not recontaminate the surface being cleaned.

Gardco suggests users always clean the instrument after each use. Depending on the frequency of use, a thin film may occur on the probe. A clean cotton swab and cleansing solution can be used to remove this.

Gardco | gardco.com

Henkel Adhesive Technologies (Dusseldorf, Germany) has expanded its production capacities in Montornès del Vallès, Spain, adding solvent- and chromium-free innovations for the pretreatment of metals in coating plants to its European portfolio. Starting this year, lubricants, cleaners and surface treatments are being produced CO₂-neutrally. In taking this strategic decision, Henkel is responding to rising customer demand and investing in continuous product optimization. 

For more than 25 years, Henkel has been supplying the chromium-free pretreatment agents Bonderite M-NT 1455T, M-NT 1456 and M-NT 10456 for coil coating to customers in various sectors, such as architecture. These agents act as an interface between the steel and the coating to provide enhanced corrosion protection and adhesion. The three solutions are now also available as solvent-free products with the suffix “SF” (solvent-free). 

“We are committed to continuously improving our products and driving innovation far beyond regulatory requirements. Developing solvent-free versions of our proven and high-performance Bonderite pretreatment agents was therefore a logical step with regard to the safety of our customers,” says Hans-Oscar Stephan, Technical Manager Metal Coil Europe at Henkel Adhesive Technologies.

One disadvantage of solvent-borne products is their boiling point of 150ºC, which leads to poorer drying of the material. In contrast, users of the solvent-free, odorless and better-drying solutions benefit from process-related advantages, such as greater product stability at higher temperatures. The solutions also have a much longer shelf life, as precipitation during storage is reduced. To meet the demand for solvent-free pretreatment products, Henkel has invested significantly in its Montornès site in Spain to expand production facilities for the polymer used in these products. This polymer gives the products their outstanding properties in terms of coating adhesion and corrosion resistance. 

Henkel also has production facilities for the automotive and aviation industries at its long-standing site in Montornès. The company says the site is therefore ideally positioned in terms of infrastructure to serve as the European hub for coil pretreatment, while also increasing capacity for global customers.

“Through this expansion in Montornès, which will serve as the new production site for Europe and the APAC region, we can ensure reliable supply capabilities to meet rising demand. This is one of the ways we are contributing to the architectural megatrend of sustainable construction projects,” says Jaime Anguera, plant manager in Montornès. 

In the future, the expanded capacities will also enable Henkel to offer tinplate manufacturers an ideal basis for converting their passivation processes. These manufacturers will have to stop using chromium in production by 2027. Henkel is thus underlining its high aspirations to achieve safer, cleaner and more efficient processes in the metals industry, as is reflected in the three pillars of its "REspect REthink REinvent" framework. The company says its goal is to drive a process of continuous optimization by further enhancing products, processes and applications, offering customers the greatest possible performance while simultaneously conserving the planet’s resources.

“We are continuously searching for ways to make metal pretreatment more sustainable for our customers, their employees and the environment. CO2-neutral production in Montornès is an important milestone as we continue to pursue this ambition moving forward,” says Volker Mansfeld, Global Vice President Metal Coil and General Industry at Henkel Adhesive Technologies.

The EcoProWet PT pretreatment system consists of a space-saving modular chamber instead of large immersion tanks. Photo Credit: Dürr

Dürr is enabling a compact and modular construction method with a system design for pretreatment in the painting process. EcoProWet PT is a flexible and scalable wet system for body pretreatment that takes less space and time to build than traditional immersion tanks. The system saves on energy and materials, and is suitable for small- to medium-production capacities.

In vehicle painting, pretreatment is still one of the process steps that takes up the most space in a plant. This is because the tanks for cleaning and electrocoating are designed for the biggest vehicle body dimensions and for the longest process times in each case. With EcoProWet PT, Dürr has developed a modular pretreatment system that is changing systems for small- and medium-capacities with a scalable, space-saving and sustainable solution.

The first station in the painting process is pretreatment, where vehicle bodies coming through from the body shop are cleaned, degreased and prepared for subsequent coating. The second process step is electrocoating, which applies a primer to the body to protect against corrosion. Previously, the RoDip rotational dip process has been used at Dürr for efficient immersion and draining processes. The EcoProWet PT pretreatment system is an alternative to the established method.

Instead of using large tanks, the system design envisages compact chambers, which are flooded or designed as spray chambers depending on the process step. At the process level, the body is loaded through a roller shutter door into the tunnel on the cross-transfer cars, which bring the body to the respective process chambers. For future series systems, up to five of these cross-transfer cars are planned, which work in previously defined sections and bring the bodies to the chambers.

Like the chambers themselves, all elements of the system have a modular design not only for efficient logistics but also for short assembly and commissioning times during construction. This applies to the tunnel, steel structure, conveyor technology and door that securely seals the flooding chamber. Because of the solid construction, it can withstand the water pressure during flooding or when the chamber is completely full. The optional insulation in the door contributes to the system’s energy efficiency by reducing the amount of heat loss to a minimum.

In the spray chamber, the bodies are treated using a large number of nozzles, which can be directed to different areas and can be flexibly switched on and off according to the size and shape of the bodies. Through this, critical areas, such as door sills, can be reached. A lance with a special rotating nozzle also promotes targeted and efficient cleaning of the interior.

In the flooding chamber, the treatment fluid is fed into the chamber from a buffer tank located above. The resulting momentum further helps to clean the body, and only as much fluid is used as needed for the respective size of the car body. The filtration process to recycle the cleaning fluid takes place in the full volume flow on the way back from the counter tank to the buffer tank.

In series operation, the cycle times and system modules are coordinated with each other in a way that the buffer tank is filled whenever fluid is needed. Because the pumps are likewise controlled in accordance with the actual need, the energy required for them to run can be reduced to a minimum.

The EcoProWet PT pretreatment system is also well suited for thin-film processes. In automotive paint shops, the previously common pretreatment process of zinc phosphating is being replaced by thin-film processes that minimize the use of environmentally harmful chemicals and the cost of wastewater treatment. These thin-film processes have become established on the market and meet the high requirements for corrosion protection.

The system was developed in close cooperation with chemical manufacturers. Together with BASF/Chemetall and Henkel, Dürr has carried out test series to validate the system concept in a purpose-built pilot system. The evaluation of the test results by the chemical suppliers is said to show how well suited the system concept is to modern, environmentally-friendly, thin-film processes. It also confirms the high flexibility of the system, which enables a process design for different substrates and body types. Through the cooperation, Dürr has developed a system that meets the sustainability requirements of future production processes.

Photo Credit: Products Finishing

Q: We’ve been running into issues with achieving a consistent color with our black oxide application, often becoming splotchy or occasionally having a reddish cast over the black finish. What would be the best way to troubleshoot our operation so that we can regain our product’s uniform coloring?

A: Black oxide coatings are widely utilized in various industries due to their excellent corrosion resistance, aesthetic appeal, and enhanced wear-resistant properties. The process involves converting the surface layer of ferrous metals into a durable, blackened oxide layer called magnetite through a chemical reaction. Despite its numerous advantages, it is not without its challenges. Manufacturers and finishers often encounter processing issues that can render subpar results and inconsistent coatings. This article delves into black oxide troubleshooting, exploring the common hurdles faced during application and providing insights into effective solutions. By understanding and resolving these challenges, professionals in the industry can achieve optimal black oxide finishes that meet the highest standards of quality and durability.

One of the first aspects to evaluate when running into issues with your black oxide application is that of your upstream or pretreatment processes. Preparing a surface prior to a black oxide application is critical to achieving a uniform distribution of color, free of splotches and inconsistencies. Alkaline or caustic cleaning solutions are primarily employed for the treatment and removal of organic contaminants; combining a product’s high pH and an elevated operating temperature (~180°F or higher) enables a cleaner to effectively remove and breakdown a broad spectrum of oils and greases that may be present on the part’s surface. Additionally, these cleaners can often contain chelating or sequestering agents that aid in minimizing select inorganic contaminants such as carbon (smut) and mild oxidation (flash rust).

The primary issue within the cleaning step is improper maintenance of the cleaning bath itself. Oil-splitting cleaners are often favored over emulsifying cleaners as they can be more economical in the long run. These products separate or split out the removed contaminants from the cleaning solution, enabling them to float and create a superficial oil layer; this imposes the potential for subsequent contamination of the cleaned parts upon removal from the solution if not properly and routinely addressed. While several methods exist to remove oil layers, overflowing weir systems are among the most effective and efficient. These systems work by constantly flowing the bath over a weir, isolating the mix of the oil layer and excess cleaner into a satellite tank that allows the two components to resegregate before returning the cleaner to the tank.

In addition to the cleaning aspect of pretreatment, surface activation or acid pickling is another crucial step influencing the resulting black oxide finish. Acid activation serves several purposes in preparing the part for blackening; first, it complements the preceding cleaning step by removing any remaining inorganic contaminants from the substrate, including rust, scales, and other metallic impurities that would otherwise inhibit the development of a uniform and adherent black oxide layer. Secondly, the acid step facilitates a controlled etch on the surface of the metal that “activates” the surface of the steel and, depending on the formulation of the acid, can attack and remove certain elemental constituents in the alloy, contributing to the promotion of a darker coloration in the subsequent black oxide step.

Often overlooked or underappreciated is the impact adequate rinsing can have on the outcome of the black oxide finish. Just as the consideration of removing contaminants is essential, so too stands the preventative effort of reapplying or carrying over pollutants between the process tanks. This encompasses previously removed contaminants and the chemistries used to do so, which can cross-contaminate the process tanks in sequence. Maintaining the rinse tanks’ cleanliness mitigates these risks by flushing the parts between active chemical steps and diluting chemical carry-over to introduce minimal drag-out into subsequent process tanks. Ideally, these tanks are continuously flowing to flush out the rinse water and prevent the buildup of pollutants that would otherwise propagate in static rinse tanks. Counterflowing rinses are often utilized as an efficient method to balance net water flow and rinsing efficacy by staggering the rinse tanks to backflow successively, offering an additional level of magnitude of cleanliness for each counterflow rinse stage employed.

Considerations for the black oxide solution are dually critical in troubleshooting issues with the quality of the surface treatment. While the chemistry and process are relatively simplistic, a few key factors should be evaluated to ensure the black oxide solution can function and develop appropriately. Maintaining adequate concentration and temperature of the operating solution ensures that the bath adequately performs to provide consistent results within and between batches.

Unlike other chemical baths that require titrations for control procedures, the concentration of the black oxide chemistry can be largely maintained indirectly through the solution’s boiling point. Black oxide chemistries are saturated salt solutions that typically possess a boiling point of around 283-288°F; maintaining the solution’s boiling point ensures that its constituents remain at appropriate ratios and stabilize the solution’s water content. As the bath operates at temperature throughout the day, water evaporates from the solution causing both concentration and boiling point to elevate. Conversely, as excess water is introduced to the solution, the concentration and boiling point depress. A best practice in maintaining these variables would be with the use of an automated water addition system, which incorporates a thermocouple and solenoid valve to gradually add water to the bath once the temperature surpasses an upper limit; this compensatory measure drives the boiling point back into operating range and restores the solution to equilibrium. Additionally, this method limits the risks of adding too much water too quickly, which can lead to boiling over of the solution.

Another aspect of temperature to be mindful of is the ability of the tank to remain or quickly regain operating temperature throughout processing. Often, people tend to overwhelm the tank with excess parts, leading to a dramatic drop in the solution’s temperature as the steel being processed acts as a significant heat sink to the thermal energy present in the solution. To counteract the drop in temperature, the heat source must regain this energy to return the chemistry to a boil. The primary concern with this scenario is the coinciding loss of convection when the boil dissipates as that is the sole source of agitation for the black oxide solution; losing the convection enables the solution to remain locally static in proximity to the substrate’s surface, providing the opportunity for free iron to deposit on the surface and limiting the ability of the conversion reaction to proceed as intended. This yields incomplete and inconsistent coloring on the surface and can contribute to a rusty appearance on the finished parts. Ideally, the solution’s temperature should not drop more than 5°F below the boiling point, which allows the bath to return to operating temperature within 1-2 minutes, depending on the method of heating and the volume of the solution in the tank; a best practice to avoid overwhelming the tank is not to exceed 1 lb of parts per gallon of solution.

In addition to maintaining the boiling point, monitoring the iron content within the solution is another factor that can significantly impact the appearance of the coloring. Soluble iron is a natural byproduct of the oxidation reaction that occurs with this chemistry and, over time, will gradually build up within the solution. Many black oxide products have an incorporated buffering capacity for iron; however, excess iron will begin to crash out of the solution once that limit has been surpassed. The convections created with the boiling of the solution stir up the iron and leave the parts being processed vulnerable to the excess iron depositing onto their surface, resulting in a red cast that is often difficult to remove. Utilizing a filter basket containing a fine stainless mesh is an effective way to passively collect and remove the excess iron from the bath during routine operation; beyond removing the iron, this method also assists in removing some of the white carbonates that naturally develop as the bath is left exposed to the atmosphere. Extending beyond the daily upkeep, periodic decant and desludging of the tank will also ensure that the chemistry is being maintained for long-term viability and continued use.

Understanding these common challenges and implementing practical solutions is the key to addressing these quality issues. Routine evaluation of these process elements, from the effectiveness of the pretreatment steps to the quality and operation of the active black oxide solution, will enable applicators to proactively identify and address issues before impacting the quality of the application. Working with your chemical provider to better understand your process and provide training for the operators will ensure that you can lower the frequency of quality issues, minimize costly preprocessing and downtime, and ultimately uphold your reputation with your customers.

About the Author

Photo Credit: Hubbard-Hall

Connor Callais

Connor Callais is an applications specialist for Hubbard-Hall. Visit hubbardhall.com.

Hubbard-Hall (Waterbury, Conn.) has launched Metal Guard 900, a viscous, solvent-based rust preventative for extended indoor and outdoor exposures on bare metal surfaces. The company says that customer usage and laboratory test using ASTM D 1748 parameters prove that Metal Guard 900 can provide greater than 3 months of water, fog and salt air protection.

Metal Guard 900 is formulated for 1-step application and removal, is reported to be safe on all metals, and lower in VOCs than other rust preventatives.

Hubbard-Hall | 855-405-6123 | hubbardhall.com

Torch Surface Technologies (a Division of Hubbard-Hall) offers a line of “heavy” zinc phosphates. The company’s TCJ zinc phosphate product line is designed to provide improved performance in corrosion protection for phos and oil and dip spin applications. The nickel-free product operates at temperatures as low as 140°F.

The company says additional benefits include ultra-low sludging (up to 75% less than traditional heavy zinc phosphates), versatility (coating weight ranges from 500 mg/ft² to more than 2000 mg/ft²), and performance that meets automotive and military specifications.

Torch Surface Technologies | 734-449-9500 | torchsurfacetech.com/pretreatments