- Email: [email protected]
Iron phosphating
IRON PHOSPHATING by Brad Gruss Pretreatment
& Process
Inc., Nicevilie,
f/a
Phosphate coatings are produced on ferrous and nonferrous metal surfaces and are composed of tiny crystals of iron, zinc, or manganese phosphates. These inorganic coatings produced on metal surfaces retard corrosion and promote better paint bonding. Phosphate coatmga are produced after precleaning or are formed in a combination hath known as a cleaner-phosphate. Phosphate coatings are generally used by the metal finishing industry for finishes such as paints. the following reasons: (I) to provide a base for hondin, ~7 organic lacquers, plastics. rubber, adhesives, etc.; (2) to provide a base of oils, waxes. and rust preventives to reduce metal corrosion: (3) to provide a base for lubricant on hearing surfaces to reduce friction; and (4) to aid in drawing and forming of metals. The primary use of phosphate coatings. however, is for the bonding of paint. Coatings produced on metal are not only stable and chemically inert toward organic finishes. hut they are also absorptive and hind organic finishes to the metal. The most important reason to use a phosphate coating is to prev’ent or retard the spread of corrosion under paint. including the areas near a ruptured film. Phosphate coatings consist of crystalline salts of the metal being treated and/or crystalline salts of metal ions added to the phosphating solution, When the metal comes in contact with the phosphating solution, pickling occurs, which results in a reduction of acid concentration at the liquid-metal interface. At this point iron is dissolvjed. hydrogen is evolved. and the phosphate coating is deposited, Should the solution contain additional metal ions such as zinc or manganese, phosphate coatings of these ions are also dcpositcd. Accelerators such as nitrite, nitrate, chlorate. peroxide or special organic chemicals may be added to increase the rate of coating deposition.
IRON
PHOSPHATING
Iron phosphating is the workhorse of the general finishing industry because of the quality delivered with regards to the newer coatings, its ease of control, nonsludging characteristics, and economics. Iron phosphate coatings are usually derived from solutions that contain very little iron. They are produced on ferrous metals through the combined use of acid phosphate salts, free phosphoric acid, plus accelerators. For nonferrous metals \uch as aluminutn or zinc. a microetched surface is produced in place of iron phosphate coating formation. Thorough understanding of available water for processing will help to minimize sludge formation in tanks and prolong tank life. Water should he analyzed for its hardness and dissolved solids in micromhos. For extremely hard waters, select alkaline cleaners or phosphate compounds with hard water stabilizers. It will be especially important for phosphate compounds to have a hard water stabilizer system built in to minimize sludge formation as well as frequent operating pH adjustment to obtain quality phosphate coating formation. Operating pH will vary widely with type of phosphate compounds and some will favor a pH range of 3.5 to 5.0, whereas others will favor a range of 4.8 to 6.0. It is more economical to use pH adjustment acid concentrate than to use phosphate compound. pH will tend to rise in operation in most instances. For combination cleaner-iron phosphates. the cleanin, (7 ic
52
Table
I. Three-Stage
PRXtXS
Time Temperature Concentration PH
Spray
Application
Methods
stage One
stage Two
Srage Three
Clean and phosphate 90 seconds 90-140°F
Rinse 30 seconds Ambient NA NeLltd
Seal rinse 30 seconds 7G140”F 0.1425% Acidic-neutral
2-4%
Acidic
coating weights and salt spray requirements for three-stage systems, but their concern should be more focused on control of soils, the cleaning ability offered. and maintenance of the system. Mixed metal lines. or systems treating nonferrous metals through an iron phosphate system. are generally faced with a major trade-off. It is very difficult to maximize salt spray performance on both ferrous substrates and nonferrous substrates running through the same line at the same time. Serious evaluation and pretesting should be done prior to the installation of a multimetal line. When high corrosion protection is required, serious consideration should be given for two pretreatment lines, especially when the finisher desires to maximize performance testing with mixed metals operations, The cleaning of nonferrous materials such as aluminum, zinc, galvanneal, and galvanired or terneplated steel require special consideration, especially with cast materials. Many die lubes proposed to be water soluble are in fact very difficult to clean. If the metal finisher does not have his or her own in-house die casting operation, then it is advised to have either a great relationship with the casting vendor or a thorough understanding of the soils used and the process utilized. Precleaners and iron phosphates for mixed metal lines are typically modified slightly to clean and deoxidize. Work with your chemical vendor to choose the right materials that offer sound cleaning, deoxidizing, and microctching in the process. without being overly aggressive. Adhesion failures occur when either the substrate is insufficiently cleaned or microetched and when there is excessive etch. THREE-STAGE
SYSTEM
1. Clewzing: The first stage is a multipurpose stage where cleaning or oil removal must take place (see Table I). Displacement of soils is produced through spray impingement and wetting afforded by the detergent packages built into acidic solutions. A fine balance of the surfactants is necessary to adequately remove these normally alkaline sensitive soils. Upon completion of soil removal, the acidic solution dissolves a minute layer of the metal substrate. A slight pH rise takes place at the substrate-solution interface causing an insoluble reaction and producing the iron phosphate coating. Typical iron phosphate coatings range from 20 to 40 mg phosphate/f? on steel substrates. The wide range of soil types and soil loads distributed unevenly across metal surfaces places severe demand on the cleaner portion of cleaner phosphate chemistries. 2. Rinsing: The continuous overflowing rinse stage is designed to flush nonadherent soils and phosphate solution from all parts. 3. Se& rinsing: This final step in a three-stage spray pretreatment removes any trace chemical residue, prevents flash rusting, and seals the pores of the iron phosphate coating. This three-stage system has several benefits. It provides for good multimetal preparation and cleaning of controlled soils. and establishes dry-film adhesion characteristics. The system. however, also has some negatives. There is no alkaline cleaning stage and the phosphate accelerator choices exhibit limited ability. Furthermore, control is difficult because of the narrow window available for it and finally phosphate shows a lack of uniformity. Nonetheless, cleaner coalers or cleaner-phosphate systems are the most prevalent spray systems for organic finishing. 54
Table
II. Five-Stage
Process Time Tempera~re
Concentration PH
Iron
Phosphate
System
stage I
Stage 2
Stage 3
ClGUl 90 seconds 90-1WF
Rinse 30 seconds Ambient NA Neutral
Phosphate 60 seconds 90-140°F
2-496
Alkaline
2-48
Acidic
Stage 4
smge 5
RillX
Seal rinse
30 seconds
30 seconds
Ambient NA Neutral
70-140°F 0.1-0.25%
Acidic/neutral
Cleaning is critical to ultimate success in the three-stage system. Acidic detergent systems are not as effective as alkaline cleaning products. This requires the manufacturer to have very tight control over incoming soil types and soil loads. The window for successful cleaning is much narrower, requiring full knowledge of incoming soils and very tight control over washer maintenance and overall cleaning efficiency. The type of iron phosphate and how the coating weight is accelerated or produced is limited in the currently available chemistries. This sometimes limits the phosphate coating and the ultimate corrosion resistance offered! Three-stage iron phosphate systems can offer high-quality results when the pretreatment chemicals are chosen for their ability to clean. FIVE-STAGE
APPLICATION
METHODS
Iron phosphate systems can utilize five or more stages, where additional stages may include additional cleaning, rinsing, and DI rinse stages (see Table II). Five-stage systems are best suited for delivering high-quality phosphate development and long-term coating life. Six-stage systems most often include a final deionized water mist rinse. I. CIenning: Cleaning, typically alkaline cleaning, produces a metal surface free of organic and inorganic reactive soils. Alkaline cleaners incorporate detergents and surfactants to wet the soil, alkaline builders to degrade, emulsify. and saponify organics. and water conditioners to soften and control contaminants. Cleaning chemistries generated with alkaline systems offer a much wider range when considering the difficulty of removal and heavier loads of soil. 2. Fresh water rinsing: The purpose of this stage is to flush all remaining organic soil from the part and neutralize alkalinity and prevent pH contamination of the next stage. Many systems do not have adequately sized rinse tank volumes or overflow capacity to effectively flush and neutralize. Part density and configuration dictate the most suitable overflow rate. Control at this stage consists of monitoring pH and total dissolved solids. 3. Phosphuting: Iron phosphating is the most common form of conversion coating in general industry for today’s new coatings. The cleaned and rinsed part enters the phosphating stage and receives a uniform acidic attack. Unlike three-stage systems where the chemistry relies both on removal of soil and phosphate deposition, the live-stage system employs a single function: phosphating. This allows a wider range of chemistries for phosphating and the ability to use ingredients that produce a much higher quality phosphate coating. Similar chemical reactions occur at the substrate-solution interface. The deposition of phosphate is not only more uniform, it is also heavier in nature and quality. Most five-stage iron phosphates deliver 40-70 mglft’ of coating. 4. Fresh wurer rinsing: The purpose of stage four is to flush any remaining phosphate solution and prevent the subsequent stage from becoming contaminated. 5. Seal rinsing: The purpose of final seal rinsing, regardless of whether it’s the final stage in either a three- or five-stage system, is to remove unreacted phosphate and other contaminants, cover bare spots in the coating, prevent the surface from flash rusting. and extend salt spray performance. There are three types of seal rinse: deionized water rinses, acidified rinses, and reactive rinses. 55
The deionized rinse is the mo\t widely used and successful over the widcxt range of paint coatings. Deionized rinses leave the least amount of potential contaminants. which may reduce the lift of the coating. The remaining two types of SeLlI rinses fall Into the nonchrome type or chromic acid type. Most honest seal rinse suppliers will claim thnt the chromic acid type of seal rinse provide the highest salt spray resistance over the widest range of paint formulations. Clnirns that \ome of the reactive nonchrome senl rin\c\ xr better than chrome can bc agreed to uith certain specific paint chemistries. The only effective way the metal finishelcan protect himself from claims is through i, qualified contmlled evaluation of the ruhstrate through the existing system and paint :tnd then alternating the seaI rinse choices. Tehting and \ubequcnt production duplication is the only way. Should your part quality dictate the use of ;L compounded chromic acid type of seal rinhe. there xe two methods for controlling the effluent (classified as a hazardous waste): conventional chemical reduction and proprietary ion exchange. The conventional reduction method reduces the total volume of waste. Thcrc xc many different existing technologies and other metals ;md ol-ganicb can lx handled. The ion exchange method is simple, cre;ites no hazurdou\ wnste, and returns all the water to process or drain.
increase
Paint Adhesion and Reduce Undetfilm Corrosion on: *Steel *Aluminum *Zinc and Galvanized Surfaces Increase Corrosion Resistance and Reduce Galling on Steel
Surface Preparation Techniques for Adhesive Bonding hy R.F. Wegman
15849
800-883-5621 www.aerocote.com E-mail: aerocote@?swbell.net Support
chemicals;
Cleaners/Degreasers, Rust/Scale Removers, Corrosion Inhibiting Oils, PassivatorslSealers Circle
56
004 on reader
information
card
$42.00
This book is intended to provide information on processes for treatment of substrates prior to adhesive bonding. Send
408 Jensen Drive, P.0:Box Houston, TX 77220
150 pages
METAL
Orders
to:
FINISHING
660 White Plains Rd. Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333.2570 All book orden mast be prepaid. PIem include SC.00 shipping ad handliig for deliveT of each book vu UPS in rhe U.S., 510.00 for each hook shipped express to Canada; and 920.00 for each book rhippsd express IO all other counrrier.
& Process
Inc., Nicevilie,
f/a
Phosphate coatings are produced on ferrous and nonferrous metal surfaces and are composed of tiny crystals of iron, zinc, or manganese phosphates. These inorganic coatings produced on metal surfaces retard corrosion and promote better paint bonding. Phosphate coatmga are produced after precleaning or are formed in a combination hath known as a cleaner-phosphate. Phosphate coatings are generally used by the metal finishing industry for finishes such as paints. the following reasons: (I) to provide a base for hondin, ~7 organic lacquers, plastics. rubber, adhesives, etc.; (2) to provide a base of oils, waxes. and rust preventives to reduce metal corrosion: (3) to provide a base for lubricant on hearing surfaces to reduce friction; and (4) to aid in drawing and forming of metals. The primary use of phosphate coatings. however, is for the bonding of paint. Coatings produced on metal are not only stable and chemically inert toward organic finishes. hut they are also absorptive and hind organic finishes to the metal. The most important reason to use a phosphate coating is to prev’ent or retard the spread of corrosion under paint. including the areas near a ruptured film. Phosphate coatings consist of crystalline salts of the metal being treated and/or crystalline salts of metal ions added to the phosphating solution, When the metal comes in contact with the phosphating solution, pickling occurs, which results in a reduction of acid concentration at the liquid-metal interface. At this point iron is dissolvjed. hydrogen is evolved. and the phosphate coating is deposited, Should the solution contain additional metal ions such as zinc or manganese, phosphate coatings of these ions are also dcpositcd. Accelerators such as nitrite, nitrate, chlorate. peroxide or special organic chemicals may be added to increase the rate of coating deposition.
IRON
PHOSPHATING
Iron phosphating is the workhorse of the general finishing industry because of the quality delivered with regards to the newer coatings, its ease of control, nonsludging characteristics, and economics. Iron phosphate coatings are usually derived from solutions that contain very little iron. They are produced on ferrous metals through the combined use of acid phosphate salts, free phosphoric acid, plus accelerators. For nonferrous metals \uch as aluminutn or zinc. a microetched surface is produced in place of iron phosphate coating formation. Thorough understanding of available water for processing will help to minimize sludge formation in tanks and prolong tank life. Water should he analyzed for its hardness and dissolved solids in micromhos. For extremely hard waters, select alkaline cleaners or phosphate compounds with hard water stabilizers. It will be especially important for phosphate compounds to have a hard water stabilizer system built in to minimize sludge formation as well as frequent operating pH adjustment to obtain quality phosphate coating formation. Operating pH will vary widely with type of phosphate compounds and some will favor a pH range of 3.5 to 5.0, whereas others will favor a range of 4.8 to 6.0. It is more economical to use pH adjustment acid concentrate than to use phosphate compound. pH will tend to rise in operation in most instances. For combination cleaner-iron phosphates. the cleanin, (7 ic
52
Table
I. Three-Stage
PRXtXS
Time Temperature Concentration PH
Spray
Application
Methods
stage One
stage Two
Srage Three
Clean and phosphate 90 seconds 90-140°F
Rinse 30 seconds Ambient NA NeLltd
Seal rinse 30 seconds 7G140”F 0.1425% Acidic-neutral
2-4%
Acidic
coating weights and salt spray requirements for three-stage systems, but their concern should be more focused on control of soils, the cleaning ability offered. and maintenance of the system. Mixed metal lines. or systems treating nonferrous metals through an iron phosphate system. are generally faced with a major trade-off. It is very difficult to maximize salt spray performance on both ferrous substrates and nonferrous substrates running through the same line at the same time. Serious evaluation and pretesting should be done prior to the installation of a multimetal line. When high corrosion protection is required, serious consideration should be given for two pretreatment lines, especially when the finisher desires to maximize performance testing with mixed metals operations, The cleaning of nonferrous materials such as aluminum, zinc, galvanneal, and galvanired or terneplated steel require special consideration, especially with cast materials. Many die lubes proposed to be water soluble are in fact very difficult to clean. If the metal finisher does not have his or her own in-house die casting operation, then it is advised to have either a great relationship with the casting vendor or a thorough understanding of the soils used and the process utilized. Precleaners and iron phosphates for mixed metal lines are typically modified slightly to clean and deoxidize. Work with your chemical vendor to choose the right materials that offer sound cleaning, deoxidizing, and microctching in the process. without being overly aggressive. Adhesion failures occur when either the substrate is insufficiently cleaned or microetched and when there is excessive etch. THREE-STAGE
SYSTEM
1. Clewzing: The first stage is a multipurpose stage where cleaning or oil removal must take place (see Table I). Displacement of soils is produced through spray impingement and wetting afforded by the detergent packages built into acidic solutions. A fine balance of the surfactants is necessary to adequately remove these normally alkaline sensitive soils. Upon completion of soil removal, the acidic solution dissolves a minute layer of the metal substrate. A slight pH rise takes place at the substrate-solution interface causing an insoluble reaction and producing the iron phosphate coating. Typical iron phosphate coatings range from 20 to 40 mg phosphate/f? on steel substrates. The wide range of soil types and soil loads distributed unevenly across metal surfaces places severe demand on the cleaner portion of cleaner phosphate chemistries. 2. Rinsing: The continuous overflowing rinse stage is designed to flush nonadherent soils and phosphate solution from all parts. 3. Se& rinsing: This final step in a three-stage spray pretreatment removes any trace chemical residue, prevents flash rusting, and seals the pores of the iron phosphate coating. This three-stage system has several benefits. It provides for good multimetal preparation and cleaning of controlled soils. and establishes dry-film adhesion characteristics. The system. however, also has some negatives. There is no alkaline cleaning stage and the phosphate accelerator choices exhibit limited ability. Furthermore, control is difficult because of the narrow window available for it and finally phosphate shows a lack of uniformity. Nonetheless, cleaner coalers or cleaner-phosphate systems are the most prevalent spray systems for organic finishing. 54
Table
II. Five-Stage
Process Time Tempera~re
Concentration PH
Iron
Phosphate
System
stage I
Stage 2
Stage 3
ClGUl 90 seconds 90-1WF
Rinse 30 seconds Ambient NA Neutral
Phosphate 60 seconds 90-140°F
2-496
Alkaline
2-48
Acidic
Stage 4
smge 5
RillX
Seal rinse
30 seconds
30 seconds
Ambient NA Neutral
70-140°F 0.1-0.25%
Acidic/neutral
Cleaning is critical to ultimate success in the three-stage system. Acidic detergent systems are not as effective as alkaline cleaning products. This requires the manufacturer to have very tight control over incoming soil types and soil loads. The window for successful cleaning is much narrower, requiring full knowledge of incoming soils and very tight control over washer maintenance and overall cleaning efficiency. The type of iron phosphate and how the coating weight is accelerated or produced is limited in the currently available chemistries. This sometimes limits the phosphate coating and the ultimate corrosion resistance offered! Three-stage iron phosphate systems can offer high-quality results when the pretreatment chemicals are chosen for their ability to clean. FIVE-STAGE
APPLICATION
METHODS
Iron phosphate systems can utilize five or more stages, where additional stages may include additional cleaning, rinsing, and DI rinse stages (see Table II). Five-stage systems are best suited for delivering high-quality phosphate development and long-term coating life. Six-stage systems most often include a final deionized water mist rinse. I. CIenning: Cleaning, typically alkaline cleaning, produces a metal surface free of organic and inorganic reactive soils. Alkaline cleaners incorporate detergents and surfactants to wet the soil, alkaline builders to degrade, emulsify. and saponify organics. and water conditioners to soften and control contaminants. Cleaning chemistries generated with alkaline systems offer a much wider range when considering the difficulty of removal and heavier loads of soil. 2. Fresh water rinsing: The purpose of this stage is to flush all remaining organic soil from the part and neutralize alkalinity and prevent pH contamination of the next stage. Many systems do not have adequately sized rinse tank volumes or overflow capacity to effectively flush and neutralize. Part density and configuration dictate the most suitable overflow rate. Control at this stage consists of monitoring pH and total dissolved solids. 3. Phosphuting: Iron phosphating is the most common form of conversion coating in general industry for today’s new coatings. The cleaned and rinsed part enters the phosphating stage and receives a uniform acidic attack. Unlike three-stage systems where the chemistry relies both on removal of soil and phosphate deposition, the live-stage system employs a single function: phosphating. This allows a wider range of chemistries for phosphating and the ability to use ingredients that produce a much higher quality phosphate coating. Similar chemical reactions occur at the substrate-solution interface. The deposition of phosphate is not only more uniform, it is also heavier in nature and quality. Most five-stage iron phosphates deliver 40-70 mglft’ of coating. 4. Fresh wurer rinsing: The purpose of stage four is to flush any remaining phosphate solution and prevent the subsequent stage from becoming contaminated. 5. Seal rinsing: The purpose of final seal rinsing, regardless of whether it’s the final stage in either a three- or five-stage system, is to remove unreacted phosphate and other contaminants, cover bare spots in the coating, prevent the surface from flash rusting. and extend salt spray performance. There are three types of seal rinse: deionized water rinses, acidified rinses, and reactive rinses. 55
The deionized rinse is the mo\t widely used and successful over the widcxt range of paint coatings. Deionized rinses leave the least amount of potential contaminants. which may reduce the lift of the coating. The remaining two types of SeLlI rinses fall Into the nonchrome type or chromic acid type. Most honest seal rinse suppliers will claim thnt the chromic acid type of seal rinse provide the highest salt spray resistance over the widest range of paint formulations. Clnirns that \ome of the reactive nonchrome senl rin\c\ xr better than chrome can bc agreed to uith certain specific paint chemistries. The only effective way the metal finishelcan protect himself from claims is through i, qualified contmlled evaluation of the ruhstrate through the existing system and paint :tnd then alternating the seaI rinse choices. Tehting and \ubequcnt production duplication is the only way. Should your part quality dictate the use of ;L compounded chromic acid type of seal rinhe. there xe two methods for controlling the effluent (classified as a hazardous waste): conventional chemical reduction and proprietary ion exchange. The conventional reduction method reduces the total volume of waste. Thcrc xc many different existing technologies and other metals ;md ol-ganicb can lx handled. The ion exchange method is simple, cre;ites no hazurdou\ wnste, and returns all the water to process or drain.
increase
Paint Adhesion and Reduce Undetfilm Corrosion on: *Steel *Aluminum *Zinc and Galvanized Surfaces Increase Corrosion Resistance and Reduce Galling on Steel
Surface Preparation Techniques for Adhesive Bonding hy R.F. Wegman
15849
800-883-5621 www.aerocote.com E-mail: aerocote@?swbell.net Support
chemicals;
Cleaners/Degreasers, Rust/Scale Removers, Corrosion Inhibiting Oils, PassivatorslSealers Circle
56
004 on reader
information
card
$42.00
This book is intended to provide information on processes for treatment of substrates prior to adhesive bonding. Send
408 Jensen Drive, P.0:Box Houston, TX 77220
150 pages
METAL
Orders
to:
FINISHING
660 White Plains Rd. Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333.2570 All book orden mast be prepaid. PIem include SC.00 shipping ad handliig for deliveT of each book vu UPS in rhe U.S., 510.00 for each hook shipped express to Canada; and 920.00 for each book rhippsd express IO all other counrrier.