Manual Phosphating

Article From: Products Finishing, from Chemetall Oakite , from Chemetall Oakite

Posted on: 8/1/2004

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Hot-high-pressure phosphating

Hot-high-pressure phosphating requires a pressure washer like this one. Pressure washers are usually fired by fossil fuels and have heating systems rated up to just below boiling.

Electric-steam generators

Electric-steam generators are the most popular type of unit for steam phosphating, although high-pressure steam boilers can also be used. Either system also relies on special steam guns that incorporate detergent valves for regulating chemical concentrations, which are mixed with steam through siphoning action, at the nozzle.

Perhaps your shop wants to get a start in phosphating. Maybe you’re tired of walking away from bidding on jobs involving large workpieces because treating them will just cost too much. Or you might want an efficient, flexible way to supplement your pretreatment at critical times. If any of these situations sound familiar, perhaps it’s time to consider manual phosphating and a pair of highly effective, straightforward manual application technologies that feature some surprising intricacies.

When Manual Approaches Make Sense

Apply a 180–210°F phosphating solution at between 750 and 2,000 psi through a washer and you have hot-high-pressure phosphating. Use a 212°+F steam, and you have steam phosphating. The manual phosphating options are just about that basic.

For hot-high-pressure phosphating, you require a pressure washer—usually fired by kerosene, diesel, propane, butane or oil—with a heating system rated up to just below boiling (212°F) and a pump rated at between 750 and 3,000 psi. For steam phosphating, the most popular type of unit is an all-electric steam generator paired with a heat-insulated steam gun. A high-pressure steam boiler can also be used simply by adding hosing and a steam wand or gun. The primary reasons for the popularity of all-electric steam generators is their lower cost and the ease with which the units can be relocated. If you already have a high-pressure steam boiler, of course, that option may be more viable.

Stated in this most basic form, those are the manual pretreatment options available to finishers. They are technologies perfectly suited for:

  • A shop that wants to get into the prepaint prep business but for various reasons isn’t ready to make the capital investments necessary to install a tunnel washer line or a soak tank system.
  • A finisher who needs or wants to find a cost-effective, practical way of treating large parts such as transformer housings, large cabinets, tanks, metal frames for the textile industry, elevator doors, HVAC fan components or fire engine/ambulance chassis.
  • A shop running at capacity or near capacity that wants to take on more pretreatment and finishing business.

But Which One?

The two choices might seem almost simple—until you ask this question: When should I use hot high pressure and when should I use steam?

Here the decision gets more difficult as you begin to compare and contrast the pros and cons of each option. Some situations recommend hot high pressure, others steam. You are invited to weigh the options in detail. Figures 1 and 2 give you a complete picture of the benefits achieved with each approach.

A good starting point, though, is that you are probably better off using hot high pressure when:

  • Soils are heavier
  • Water is harder
  • Fossil fuels are inexpensive
  • Parts you plan to hang or rack for treatment can be easily secured

On the other hand, manual steam phosphating is the technology of choice when:

  • Soils are lighter and chemical, fuel and water conservation is key
  • Water is soft or softened
  • Release or treatment of large quantities of spent solution are potential concerns
  • Overspray, open flame or fossil fuels are key personnel concerns

Hot-High-Pressure Manual Phosphating

If you know someone in the business of manual phosphating or have looked into the concept before, chances are that you think of hot high pressure as synonymous with manual phosphating. It is the more traditional approach, perhaps most attractive because the physical force of the spray can blast off soils. The units generally incorporate fossil fuel powered motors driving a high-pressure pump and a horizontally fired burner system. Hot-high-pressure phosphating generally requires special drainage, recycling, and racking provisions. If you plan to go with this approach, therefore, you will need to:

  • Dedicate an isolated section of your facility with good drainage.
  • Make sure you have the capability, facilities and permissions to treat/release/ hold/haul spent phosphating and rinse effluent.
  • Have a system or plan for racking, hanging or positioning parts for thorough cleaning and phosphating.

FIGURE 1: Hot-High-Pressure System

Benefits
Heavy soil removal
High impingement
Handles hard water well
Less streaking due to dry-down
Good rinsing
Faster
More forgiving
No hose length limitation

Minimum Requirements
3.5 – 5 gpm
700 – 3,000 psi rinse
230 – 440 volt 3 pH motor
750 – 1,200 psi chemical application
7.5 – 10 hp motor


FIGURE 2: Electro-Steam Generator System

Benefits
Low chemical use
Low water use
Low disposal costs
All electric
No venting required
No open flame
Safer to use—no splash
Little problem with hanging parts
Few moving parts
Low maintenance

Minimum Requirements
1.0–3 gpm
60–100 psi rinse
230–440 volt 3 pH motor

Alternatively, you can purchase and install a closed-loop spraying system—similar in appearance to a paint spray booth— that partitions off overspray and collects it for disposal.

Electro-steam is less recognized as a phosphating option and should not be confused with the near-steam pressure rinsing accomplished with some pressure phosphating units. Hot-high-pressure units generally spray hot solutions at five gpm. Sometimes nozzles are switched and an adjustment made in the regulator so that the unit is operating at something like 2.5 gpm. With only half the volume moving through the heating coil, you get superheated water, not true steam.

For rinsing, the approach is pretty foolproof. To make it work for phosphating requires additional adjustments of the chemical injectors. Balancing the concentration for consistent conversion coating can be a problem. It can require a lot of operator finesse.

Manual Steam Phosphating

Electric-steam generators, like the hot-high-pressure units, come factory assembled and wired. They incorporate automatic controls and key safety features such as steam outlet and blow down valves, water feed line check valves and gages with glass and guard rods. Units are constructed under American Society of Mechanical Engineers' Boiler and Pressure Vessel Codes. Steam guns commonly used with the generators incorporate detergent valves for regulating chemical concentrations, which are mixed with the steam, through siphoning action, at the nozzle end. Insulated grips keep operators’ hands relatively cool during application of steam solutions.

Perhaps the most significant difference between a hot-high-pressure unit and a steam generator is the throughput of chemicals. While a typical hot-high-pressure washer will consume 3–5 gpm and apply it with great force, an electro-steam system will consume only one gpm, applied in a gentler stream.

Because there is less solution sprayed and lower pressures, there is less effluent to deal with and less of a problem with overspray and impingement on parts, so a physical setup for steam phosphating usually does not have to be as elaborate:

  • A relatively small corner of your facility will usually work fine.
  • The treatment, release, holding, and/or hauling of spent phosphating solutions are less than half that encountered with hot-high-pressure.
  • Racking, hanging or positioning of parts is usually easier with steam phosphating.

Selecting and Applying the Chemicals

Once you’ve settled on a manual phosphating approach and identified where you’re physically going to do the work, you need to:

  • Select the appropriate chemical formulations.
  • Use the appropriate techniques for applying the cleaning and phosphating solutions and rinsing.

What Chemistry?

Selection of the appropriate cleaner and/or cleaner/phosphate can be daunting, especially in a startup situation, but here are some general guidelines.

If parts are heavily soiled, a separate cleaning step may be required. Depending on the nature and severity of the soils involved on workpieces, the pre-cleaner should be a powdered or liquid alkaline cleaner or a liquid alkaline/solvent blend of wetting agents, water miscible solvents and other active ingredients formulated for light- to heavy-duty cleaning. We generally recommend multi-metal cleaners that can be used safely on steel, brass and copper, or aluminum or mixes thereof.

For hot-high-pressure cleaning, the wetting agents should be low foaming. For steam cleaning, the components should be selected for their ability to work effectively at the elevated temperatures and to avoid the rapid dry down that can result in hazy, cloudy or streaky surfaces and hinder the subsequent finishing.

If soils are light, a combination cleaner-phosphate can be used. This reduces the processing time and volume of wastewater generated. The traditional approach has been to use a molybdenum-based (moly) three-stage iron phosphate with an intervening rinse. The trend today is to use more active accelerators—chlorate-based chemistry, for example. This approach produces good cleaning, quick coating and minimal flash rusting.

No-rinse, moly-based phosphates are also available where rinsing is not an option. If you are faced with federal or state/local limits on molybdenum, then you can turn to moly-free one-step cleaner/phosphates. But a water rinse is always recommended where practical. If your shop is located in an area with very alkaline water, then you may also want to use an additive to reduce pH. If your water is very hard, other additives can be obtained to condition the water to reduce or eliminate scale buildup.

Lastly, a final rinse may be used to increase paint adhesion and corrosion resistance. This is the beauty of manual phosphating—the processing steps can be easily changed from job-to-job to produce the varying/desired results.

Consult your chemical supplier for recommendations on the products more appropriate for your situation.

Cleaning and Rinsing Rules

While manual phosphating is quite a creative art, there are some basic rules it is wise to follow, especially if you hope to get top-quality paint or powder coating results through flawless manual pretreatment.

Here are just a few of the most important:

  • Always clean and phosphate from the bottom up.
  • Rinse from the top down.
  • Follow recommended concentrations and titrate often.
  • Take your time cleaning, but if you need to rinse, do it quickly and thoroughly.
  • Before phosphating, wipe the cleaned or rinsed surface with a white glove or cloth and check the cloth for soils that might remain. Clean and/or rinse again if necessary.

Just Do It!

Whether you want to get a start in phosphating, are tired of walking away from bidding on treating large workpieces or want to find a flexible, efficient way to supplement your primary pretreatment operations, manual phosphating can be the key. Start simply and, by choosing the right technology and keeping a few guidelines and rules in mind, you can be very successful and competitive. So choose wisely!

Hot-high-pressure phosphating requires a pressure washer like this one. Pressure washers are usually fired by fossil fuels and have heating systems rated up to just below boiling.

Electric-steam generators are the most popular type of unit for steam phosphating, although high-pressure steam boilers can also be used. Either system also relies on special steam guns that incorporate detergent valves for regulating chemical concentrations, which are mixed with steam through siphoning action, at the nozzle.

When doing hot-high-pressure phosphating, it may be advisable to purchase and install a closed-loop spraying system called a “Pressure Island” to partition off overspray and collect it for disposal.


 


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