Preparing Steel for Organic Coatings

Article From: Products Finishing, from Pretreatment & Processes Inc.

Posted on: 1/1/2000

Metal preparation for steel can be broken down into four areas: cleaning, rinsing, phosphating and seal rinsing.

Metal preparation for steel can be broken down into four areas: cleaning, rinsing, phosphating and seal rinsing. Of these, cleaning is the single most important function related to pretreatment.

What are the options available to clean and conversion coat steel fabrications prior to organic finishing with the least amount of negative environmental impact? The marriage between the type and quality of the pretreatment and the type and quality of the coating has a direct bearing on the ultimate quality of the finished product.

With today's regulations concerning water, air and worker safety, the burden of performance is directly pointed to maximize cleaning and pretreatment, or in other words, to find the best combination of pretreatment and organic coating that meets the user's quality needs and does so within a given dollar budget with the least environmental impact.

 


TABLE I—Cleaner Selection
Alkaline Cleaner Characteristics Choice Relationship
High alkalinity
pH 11-13.5
Non-caustic sensitive metal processing only. Ferrous, stainless, and yellow metals. Composites.
High alkalinity (buffered)
pH 10.5-12.5
Multi-metal lines ferrous/non-ferrous.
Low alkalinity
pH 6-9
Non-ferrous, aluminum, zinc, also effluent sensitive or restricted cleaning operations.
Surfactants
High-foam surfactants Static tank cleaning, immersion systems, and spray wand non-recirculating systems.
Controlled-foam surfactants Agitated immersion ultrasonic, turbulator.
Low-foam surfactants Spray cleaning and recirculated systems.
Temperature - Time
High temperature
140F and above
Aged and oxidized soils of a waxy nature, heavy accumulations and short process contact time.
Medium temperature
120-140F
Controlled soils. Light to heavy accumulations. Short to medium contact time.
Low temperature
90-120F
Light to medium soils. Typical for medium contact time.

 


Cleaning

Historically, spray alkaline cleaners have been the best choice for providing clean, water-break-free fabrications.

Listed in Table I are the choice relationships typically made in cleaner selection when considering metals, soils, temperature, surfactants and contact time.

Traditionally, cleaners have relied heavily on the choice of medium- to high-alkaline salts. These include:

Alkaline Builder
Potassium Hydroxide
Sodium Hydroxide
Sodium Orthosilicate
Sodium Metasilicate
Trisodium Phosphate
pH(+)0.5 pct
13.0
12.7
12.6
12.0
11.8

The primary purpose of alkaline builders in cleaners is to react with soils to modify, digest, disperse, and/or suspend them, to condition water, and to prevent re-deposition of soil.

Surfactants, or detergents, have always been the "key ingredient" of cleaners, as wetting must be accomplished or thorough cleaning will never take place!

Surfactants lower the surface tension of the soil to the metal and the soil to the solution. In order for cleaning to take place, surfactants or detergents must "wet" the surface. It is this affinity of the cleaner solution to reach the base metal that wets or displaces the soil barrier film. Cleaner manufacturers offer a broad range of proprietary surfactants.

Water Conditioners. Water conditioners are also key to cleaner composition. Typically they are phosphates, sequesterants and chelates. The purpose of their inclusion is to soften water, make cleaning more effective and prevent the precipitation and deposition of calcium, magnesium and other minerals that become a portion of the heated alkaline solution.

The most effective of the water softeners are also the most detrimental to effluent considerations. Chelates are not selective and also tie up restricted metals and prevent easy precipitation.

To date, the three areas most affecting effluent with regard to cleaners have been:

  1. pH
  2. Total oils/organics
  3. Restricted metals

Increasing legislation and tightening of limits can be expected in these three primary areas. Additionally, the burden on cleaners has been directed at reducing or eliminating chelates that affect the removal of heavy metals, reducing or eliminating the medium- and high-alkaline builders, which affect pH, soil splitting and neutralization costs.

 


TABLE III—Water Preferences for Different Applications
Alkaline Cleaner Raw if scale/scum are not a problem. Blend D.I./Raw, or softened.
Intermediate Rinse Raw (lowest cost, scale/scum should not be a problem here.)
Phosphating Distilled/R.O.
Raw
Blend D.I./Raw
Seal Rinse Distilled/R.O.; Raw is second choice, never softened.
Makeup Water for all of the above D.I./R.O.
Raw
TABLE II—Water Comparison
  Raw Softened D.I./R.O.
Impurities Varied amounts of minerals including hardness depending on the water source. Varied amounts of minerals including hardness depending on the water source, low calcium and magnesium. Practically none.
TDS Dependent on source. Somewhat higher than the raw water. Near zero.
Advantages Least corrosive, lowest cost. No scale/scum problems. No scale/scum problems. No residue after drying. Increased salt spray.
Disadvantages Scale and soap scum, residue after drying. Slightly more corrosive than the raw water, less visible residue after drying. Not for phosphate or seal rinse stages. Most corrosive (by itself.) Highest cost.

 


With the added responsibility of reporting hazardous incoming ingredients, which is required as of October 1, 1993 under SARA Title 313, one can only summarize this as the development of individual baselines for further reductions in the use and handling of so-called hazardous ingredients. The EPA's 33/50 program of reducing toxic chemicals by 50 pct by 1995 is voluntary (today)! Coupling this with localized phosphate restrictions for industrial products finishers, the impact on alkaline cleaners as we know them are tremendous!

On the issue of cleaning, what can you do? Actually there are several things:

  1. Carefully monitor and limit incoming soils in areas which affect difficulty of removal. Minimize heat-sensitive soils, those containing waxes, paraffin and chlorinated hydrocarbons.
  2. Carefully monitor and limit soils that you apply in-house. Reduce and control volumes, and viscosities, using no more than necessary. Change storage practices of fabricated metal products and minimize the need for corrosion inhibitors.
  3. Choose soils that are easily cleanable, splittable or decantable.
  4. Use higher-quality cleaners that can be used at lower concentrations, yet offer excellence in cleaning, tank life, and cost usefulness.
  5. Investigate additional mechanical improvements to process washers that increase impingement, reduce concentration and provide oil-skimming efficiencies.
  6. Monitor incoming metals/alloys for heavy metals.
  7. Carefully monitor chemical pretreatment suppliers' MSDS information. Search for cleaners that are:
  • Non-Caustic
  • Non-Chelate
  • Non-Phosphate
  • Non-Reportable

Rinsing

Pretreatment solutions typically make up two to four pct by volume of the total solution, with 96-98 pct of the raw water source as the balance. Rinse water is important when considering that poor raw water quality reduces corrosion and humidity protection of coated parts. Your water quality prior to mixing with chemicals and prior to using as a rinse between chemical steps is very important.

The first step in evaluating a process for pretreatment is knowing what makes up the water source. Tables II and III provide a plus/minus general comparison on water choices, along with water preference for the four major sub-groups of a pretreatment process.

Regulations and good judgment have encouraged the finisher to conserve, re-use and treat water in order to minimize costs and reduce the burden of negatively affecting our raw water aquifers.

What can the finisher do to rinse effectively, but minimize water usage?

  1. Reduce chemical bath temperatures to minimize evaporation. For example, reduce 150-160F to 90-140F or lower, if possible.
  2. Understand that product design plays a major role in rinsability, water cupping, and stage-to-stage carryover.
  3. Evaluate racking modifications.
  4. Make mechanical modifications to the rinse stages of a washer, investigate backflowing, fresh water misting nozzles, and multiple use of raw water.
  5. Investigate DI water, and regeneration of raw water through RO or
    ultrafiltration.

Phosphating

Historically, the choices for conversion coating on ferrous substrates have been iron phosphate or zinc phosphate.

The advent of higher-quality finishes such as high-solids, electrocoat and powder, coupled with environmental issues, has begun a trend to eliminate heavy metals.

Prepaint zinc phosphate is widely specified for automotive-related finished parts. The trend in appliance and general industrial products is toward high-quality iron phosphate wherever possible, as iron places less demand on post treatment processes and costs.

Phosphating develops a non-conductive layer between the base metal and coating, increasing the life of the coated product.

It should be noted that iron phosphate increases dry film adhesion properties on non-ferrous metals, but offers substantially reduced corrosion protection for these metals when subjected to high humidity and a corrosive environment. Careful pretesting should be done for mixed metal lines or processes exclusively for non-ferrous metals.

On the issue of phosphating, the areas most negatively affecting finishing are:

  1. There are some localized geographic bans on the use of any form of phosphorus.
  2. Phosphoric acid is a listed ingredient and must be reported when used in amounts of 10,000 lbs or greater. If your corporate goal is to eliminate all listed ingredients and you wish to paint metal, you are in trouble as conversion coatings require some form of free acid to begin the phosphating reaction.
  3. The accelerators used in phosphate solutions to speed up the deposition or increase the amount of iron phosphate may come under regulatory fire. Molybdate, a very common ingredient in both cleaner-coaters and straight iron phosphates, has been targeted and is now being restricted in a few geographic locations.

On the issue of phosphating, what can the finisher do? Actually there are several things:

  1. Check with your state or local municipality for any pending phosphate bans. If you are currently in a geographic area not allowing phosphate, the choices are alkaline clean or non-phosphate acidic cleaning and preferably DI rinsing. The concerns will be flash rusting (due to lack of iron phosphate) and coating performance. The coatings may have to be upgraded to meet your specifications.

Seal Rinsing

A reactive material will either chemically react with the substrate and physically improve the results, or the reactive portion of the material may bond to the substrate without a chemical reaction. The two major types of reactive seal rinses are heavy-metal-containing: chromium, zinc, copper, molybdate, others, which are applied, used and treated or dried in place.

The second reactive type is the organic family which may utilize polymeric, water-soluble resins, and modified carboxylates. Reactive organic material may either bond to the substrate or chemically react and bond.

The non-reactive chemistries include acidified rinses, and deionized water. Non-reactive acidified rinses offer the tendencies of reducing or eliminating flash rust, but accomplish little in terms of increasing corrosion resistance.

Deionized water offers the greatest benefit to the widest range of coatings. DI final rinses flush away hard water contaminants, and leave the substrate in a slightly acidic state.

Concerning the issue of seal rinses, what can the finisher do?

  1. Re-define current coating specifications. Are they outdated or written with the luxury of chromium-bearing chemistries in the pretreatment or paint?
  2. Test new chemistries that are metal free.
  3. Incorporate DI water final rinse options.
  4. Raise the quality of phosphate deposition, allowing less demand on final rinse characteristics.
  5. Evaluate higher-quality, more consistent metal substrates.
  6. Evaluate newer technology coatings: electrocoat, high-solids, powders.

Summary

Preparing steel for organic coatings while minimizing the environmental impact requires paying more attention to both current legislation and future trends in restrictions. It is critical to stay abreast of new chemistries that enhance and simplify post treatment of effluents.

Review with your pretreatment vendor the current state-of-the-art technologies in both soils application and soils removal. Whenever possible, look closely at non-hazardous materials. Then develop a sound pretreatment process and control that process. PFD

Comments are reviewed by moderators before they appear to ensure they meet Products Finishing’s submission guidelines.
blog comments powered by Disqus



Suppliers | Products | Experts | News | Articles | Calendar | Process Zones

The Voice of the Finishing Industry Since 1936 Copyright © Gardner Business Media, Inc. 2014

Subscribe | Advertise | Contact Us | All Rights Reserved