Metal Pre-Treatment. Information for industrialists, marketing. specialists, technical designers, managers and buyers...

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1 Metal Pre-Treatment British Surface Treatment Suppliers Association Information for industrialists, marketing specialists, technical designers, managers and buyers...

2 Pre treatment Why use pre-treatment? The performance of all paint and powder coated finishes depends on the correct preparation of the metal substrate. Failure of coatings in service with problems such as corrosion and blistering are nearly always caused by inadequate pretreatment. The pre-treatment is often more important than the final finish. As well as providing corrosion protection pre-treatment can offer improvements in wear resistance and other valuable properties. The most widely used pretreatment is known as Phosphating. This is also defined as a Conversion Coating. Conversion coatings are the result of a chemical reaction between a metal and a chemical solution. In addition to Phosphating there are other pre-treatment conversion coatings which can be used on aluminium, magnesium and zinc alloys. Pre-treatment offers significant benefits in a number of key areas, for example: Consistent quality of coatings to meet specification requirements. Significant improvements in paint and powder coating performance by improving corrosion resistance under the organic coating surface as well as providing increased adhesion and improved flexibility. Adhesive bonding and laminating is made possible by specific pretreatment. Heavy Phosphate coatings make it possible to draw and form metals by the production of high levels of surface lubrication. Wear resistance and anti-galling properties for critical moving parts. Improvement in corrosion resistance of metal parts by providing a base for waxes and rust preventing oils Introduction Metal pre-treatment is a hidden technology that benefits everyone. We all require quality, performance and safety from the products that we buy, the buildings we work in, and the cars that we drive. The phosphating of metals meets many of these requirements in a wideranging and effective way and is the most widely used form of metal pretreatment. The first reliable record of a phosphate coating being applied as a means of corrosion protection is found in a British Patent granted to William Ross in The patent covered the treatment of iron corset stays, which could be used as a substitute for whalebone. According to the patent the iron stays were prevented from rusting by plunging them red hot into phosphoric acid. Phosphating as we know it today is attributed to the work undertaken by Thomas Watt Coslett of Birmingham. Coslett s original patent, dated 1906 covered the treatment of iron and steel by immersion in a solution of phosphoric acid and iron filings. The process operated at the rolling boil and components were immersed in the solution for 2.5 hours. The resulting iron phosphate conversion coating provided improved corrosion resistance. This work was the beginning of continual research and development to refine and enhance phosphate processes. Today the technology can be applied by spray or immersion. Process times have been reduced to one to two minutes and operating temperatures as low as 30 Centigrade are employed. Painting and powder coating of metal parts requires metal preparation in order to ensure adequate corrosion resistance, good appearance and adhesion. During phosphating the metallic surface is actually converted to a phosphate coating. A chemical reaction between the coating solution and the substrate results in a very slight dissolution of the metallic surface by the coating solution, and the resultant conversion of that surface to the phosphate coating. Painted steel test panels with & without pre-treatment after salt spray testing Steel test panel without phosphate coating Steel test panel with phosphate coating 2

3 Metals to be coated Phosphating of steel and zinc The most common surface preparation prior to painting is either an iron or zinc phosphate coating. In general, if a metal will react with phosphoric acid, it can be assumed that it will accept a zinc phosphate coating. Metals such as stainless steel, lead, tin, and nickel will not react with phosphoric acid and will not accept a zinc phosphate coating. A typical phosphate coating structure However, the most common metals are cold and hot rolled steel and various forms of galvanised steel. The phosphate coating reaction can be characterised as an acid-base reaction including the dissolution of the metal surface by the phosphoric acid. As this dissolution occurs, the ph at the metal surface and chemical solution interface increases. The metallic phosphate becomes insoluble at the surface and forms a layer on the surface of the metallic component. Special additives are used to promote the coating deposition on different metals and to increase process efficiency. For example, colloidal suspensions of titanium phosphate compounds are applied to the surface of both ferrous and nonferrous metals to promote crystal refinement (build very small crystals), and also to provide growth sites. These same colloidal suspensions are also employed to promote crystal growth on galvanised steel and aluminium surfaces. Additives such as nickel, manganese and organic compounds are added to phosphate baths to accelerate coating formation, modify crystal morphology and improve performance. Fluoride is added to facilitate coating deposition on aluminium and other substrates. In addition to ferrous and nonferrous metals, plated surfaces can also be phosphated, to improve the adhesion of organic coatings to the plated surface. While phosphate coatings provide little or no protection against white corrosion of zinc or zinc alloy plated surfaces, they do promote adhesion of organic coatings to the surface, thus improving the long-term corrosion resistance. In phosphating plated surfaces, it is important that those surfaces not be chromate passivated, or the phosphate reaction will not occur. Zinc and cadmium plated surfaces will form a phosphate coating, whilst nickel, tin, and chromium plated surfaces cannot be processed satisfactorily. Alloy deposits, such as Zinc/Nickel and Tin/Zinc can be phosphated. The quality of the coating depends upon the content level of the alloying metals, and the uniformity of distribution of the plated deposit. Aluminium The use of aluminium in manufacturing is becoming increasingly important. It is the most abundant metal in the world and offers significant advantages. These include low weight, high strength, good corrosion resistance and formability. As a result of these benefits aluminium is used extensively across a wide range of manufactured products. The major applications are in the aerospace, architectural, automotive and packaging industries. Conversion coating technology has been developed to meet the demands of these markets. Important recent developments have been driven by a need to provide good corrosion resistance together with environmental benefits. The automotive industry needs to increase passenger safety and reduce vehicle weight to enhance fuel economy. It also requires processes which make it easier to recycle materials at the end of the life of the vehicle. Conversion coating technology has assisted the growth of the use of aluminium by improving effective adhesive bonding. Modern pre-paint technology is also designed to improve the ability to recycle. The aerospace industry depends upon lightweight, strong and corrosion resistant aluminium components. Without this important metal rapid and cost-effective air travel would not be possible. Pre-treatment technology provides the basis for exceptional corrosion resistance, low electrical resistance and improved adhesive bonding to aluminium. Architecture is a major market for aluminium as the metal allows for innovative and artistic designs, reduced energy requirements in buildings and strength in design. The essential long life required of aluminium products in architectural applications is totally dependent on carefully applied pretreatment technology. Extensive research work is continuing to provide benefits in all these sectors and at the same time to ensure that the surface finishing industry responds to increasingly demanding environmental requirements. Chemical suppliers should be consulted on the selection of newly developed coatings for aluminium, magnesium and other metals that avoid the use of hazardous materials such as hexavalent chromium. 3

4 Phosphating Process Selection Several types of phosphate process are available. Selection depends on the performance requirements of the end product. Each process type has been developed to meet specific applications and needs. Specifiers are strongly advised to consult suppliers of process chemicals for advice on the type of coating most suitable for their needs. Lightweight amorphous iron phosphates are generally selected if the end product requires excellent paint adhesion with relatively minimal corrosion resistance. The process has low chemical costs. It is easy to control and has low process heating requirements. Lightweight (microcrystalline) zinc phosphates (crystals less than 5.0 micron in diameter) are generally employed where higher corrosion resistance is required, whilst still retaining very good adhesion and mar resistance of the painted finish. Heavy macro-crystalline zinc phosphates are selected for applications such as fastener coatings, or wire and tube drawing aids. In wire and tube drawing and cold heading operations, the zinc phosphate serves a similar purpose as in painting operations, except that in this case the phosphate is being used to provide excellent adhesion of Components treated with manganese iron phosphate drawing lubricants such as soaps and/or oils. Manganese Iron phosphates and calcium zinc phosphates are often used to provide break-in and to reduce galling and pick up of mating surfaces such as valve train components or hypoid gears. These heavy weight coatings have good oil absorbing properties and can offer good corrosion resistance when used with a rust preventing oil. Military ordnance components are often finished by this method. In break-in applications, the function of the phosphate coating is to retain the lubricating oil between the two mating surfaces, long enough for the surfaces to mate. Usually the phosphate remains intact for less than one minute, however this is long enough for parts to mate without galling or scoring. Phosphate coatings are always intended to be employed as an intermediate coating not as a Product treated with lightweight iron phosphate decorative finish. As a result phosphate coatings are used as a key for some other, subsequent coating such as paint, oil, lubricants, adhesives, etc. Because phosphate coatings are so versatile, the range of applications for which they can be employed is very large. In addition, the method of application of the phosphate coating can be varied. Phosphate coatings can be applied to parts based on both the size and shape of the part as well as the type of coating being deposited. Some coatings, such as heavy zinc phosphate and manganese phosphates are only applied using immersion techniques; whilst the microcrystalline zinc phosphates and iron phosphates, can be applied using either spray or immersion techniques. The application method used in the case of lightweight iron and zinc phosphates depends on several 4

5 Phosphating Process Selection (cont.) critical factors. The type of part being processed, the quantity and size of parts being processed, and the type of coating required. For example, large sheet metal parts such as domestic appliances and office furniture are more efficiently processed through conveyorised spray plants, if they are to be wet painted or powder coated. If, however the parts are to be immersion painted through a process such as electropaint, it is more efficient to rack them for immersion processing, so they can be immersion phosphated and painted without re-racking them. As it is not economical or practical to rack very small parts, such as fasteners, which may be processed in very large volumes, these parts are most often processed through bulk processing systems such as Barrel Phosphate Lines. These barrel processing lines are similar to barrel plating lines and can process very large volumes of small parts such as small brackets and hardware, with minimal handling. The type of phosphate conversion coatings commonly used can be categorised as shown in the table below. Type Zinc phosphate oiled components Calcium zinc phosphated components Mineral Formula General Coating Name Weight Appearance Lightweight Vivianite (FeO) x (FePO4) y 0.3 to 0.9 g/m 2 Iridescent Pink to Iron Phosphate Yellow, to Blue, to Light Grey. Microcrystalline Hopeite or Zn 3 (PO 4 ) 2.4H 2 O 1.4 to 2.7 g/m 2 Light Grey Zinc Phosphate Phosphophyllite FeZn 2 (PO 4 ).4H 2 O Macrocrystalline Hopeite or Zn 3 (PO 4 ) 2.4H 2 O 7.5 to 30 g/m 2 Light Grey Zinc Phosphate Phosphophyllite FeZn 2 (PO 4 ).4H 2 O Manganese Iron Hureaulite (Mn,Fe) 5 (PO 4 )4.H 2 O 7.5 to 30 g/m 2 Dark Grey to Black Phosphate Calcium Zinc Scholzite CaZn 2 (PO 4 ) 2.2H 2 O 1.6 to 6.5 g/m 2 Light Grey Phosphate 5

6 Typical Phosphating process sequence The basic cleaning cycle is:- Clean Effective cleaning is critical, as a good even phosphate reaction will only occur on a surface that is chemically clean. Cleaning procedures include water soluble products based on alkalis or acids, solvent cleaning, bead blasting and mechanical abrasion. The acidic phosphate solution must be able to react with the substrate (iron in a steel surface, zinc in a galvanised surface etc). Residual soils, oils, scales or oxides will, inhibit this reaction unless they are completely removed. Components that are heavily scaled or oxidised will require additional processing through acid pickles usually based on hydrochloric, sulphuric or phosphoric acids before being phosphated. Rinse Surface conditioning Thoroughly in water after cleaning to remove all residual chemicals. Excessive alkaline or acid carryover will interfere with the subsequent phosphate reaction resulting in patchy coatings and heavy sludging of the solution. Generally after strong alkaline cleaning or acid pickling the subsequent phosphate coating becomes coarser with the formation of larger crystal structures. To eliminate this effect grain refiners are used, either included in the phosphating solution or as a separate pre-dip immediately prior to the phosphating operation (e.g. colloidal titanium phosphate used prior to zinc phosphating). In spray operations grain refiners can also be included in a suitable alkaline cleaning stage. It is essential to consult both chemical and plant suppliers on the precise selection of all processes associated with phosphating. Rinse Post Phosphate Rinses All traces of phosphate solution must be removed. Any residues left on the component at this stage can result in accelerated corrosion and/or blistering of subsequent organic coatings. Final rinsing using mains water may also leave behind hard water salts which can effect the coatings final performance. To eliminate this, final rinsing in demineralised water and/or the use of suitable post phosphate rinse may be advised. Can be used as a final process stage to remove the effect of residual hard water salts or to enhance performance. 6

7 Typical Phosphating process sequence (cont.) The benefits offered by post Phosphate rinses are: To inhibit blistering during exposure to humid exposures. Provide a degree of in-plant corrosion resistance. Enhanced corrosion resistance resulting from a chemical reaction with the phosphate conversion coating. Until recently most passivation processes involved the use of soluble chromium salts. Important product development has made available chromium free inorganic and organic polymer based products offering good performance and avoiding the environmental problems involved with hexavalent chromium. Final Finishes It is important to understand that phosphate coatings provide little or no corrosion resistance until painted or sealed. It is recommended that the final finish is applied without delay. If the phosphate coating becomes contaminated by airborne dust and moisture this can have a dramatic impact on the life of the treated component. In order to protect the phosphate coating if parts are to be stored, even for a short time, additional protection is generally required. Modifications to the basic process cycle described may be necessary. Factors such as process type, performance requirements, substrate condition and available space can have a big impact. The most suitable process for each application is best recommended by chemical suppliers. Micro crystalline zinc phosphated domestic appliances Micro crystalline zinc phosphated office furniture 7

8 Process plant Mission Statement The British Surface Treatment Suppliers Association is dedicated to the promotion and continuous improvement of the surface treatment industry to the mutual benefit of its members and the customers they serve. Members are committed to upholding the highest standards of quality, health, safety and environmental protection. Aims & Objectives: To expand the market for surface treatment products. To promote the benefits of surface treatment products and processes to manufacturing industry. To promote development and use of innovative surface treatment technology. To encourage higher standards of quality in the surface treatment industry. The Surface Engineering Association The SEA represents the interests of leading trade associations in the surface finishing industry. Expert technical advice is freely available on the SEA Help Line on or by on An answer for every question The Surface Engineering Association is a centre for information on coatings and corrosion prevention. We can help you get in contact with specialised suppliers of products and services. 8

9 Process plant Spray systems are used for large sheet metal parts such as automotive parts and domestic appliances. These systems can include a wide variety of process stages, dependent upon the process, the shape of the components, capital available, environmental and cost considerations. Three Stage Plants If there is a requirement for low capital investment, limited floor space, and minimal performance requirements, then a three-stage, detergent iron phosphate (sometimes referred to as Cleaner-Coaters) can be considered. In this case the phosphating solution contains surfactants to provide the required cleaning properties. Three stage systems are employed when the components entering the finishing system are reasonably clean. These systems are not suitable where high corrosion Phosphating of zinc coated car body resistance of the finished product is specified. In these systems the normal sequence is: Stage 1: Clean and Phosphate Coat. Stage 2: Water Rinse. Stage 3: Final Sealer/De-ionised water rinse. Followed by drying and finishing. Automatic process control dosing equipment The next level of sophistication is the expansion of the process to include a separate cleaning stage and a separate phosphating stage. The advantages of this process are improved cleaning, and as importantly much longer cleaner life, with much less solution to be waste treated. In addition, the phosphate solution is infrequently dumped which also reduces effluent volumes. The separation of the cleaner and the phosphate solution improves control, cleaning and improves quality. The process for this system can be either an iron or a zinc phosphate. In selecting an iron phosphate, environmental discharges will not contain heavy metals for further waste treatment, but the performance will not be equivalent to zinc phosphate. The performance and operating cost, however, will be superior to the three-stage system. 9

10 Process plant A typical five-stage plant consists of: Stage 1: Alkaline cleaner Stage 2: Water rinse Stage 3: Phosphate solution Stage 4: Water rinse Stage 5: Sealer/de-ionised water rinse followed by drying and finishing. Beyond the basic five-stage system, there are many options available. Double cleaner stages with recirculation to improve efficiency. Two cleaners, with a rinse following each, to allow the operation of two different cleaners if needed for specific soil removal. In all cases where water rinsing is specified the use of counter-flow rinses is desirable to minimise water consumption and effluent levels. Water rinses following the sealer stage, to prevent sealer drag-in to electrocoat paint tanks. In some cases these are counter flowing rinses so the last rinse can be of high purity, such as de-ionised water. With the increased use of electrodeposited paints and powder coating, the most widely used process involves double cleaning, a grain refiner application zone, and double final water rinses. Stage 1: Alkaline Cleaner Stage 2: Alkaline Cleaner Stage 3: Counter flowing rinse Stage 4: Grain Refiner rinse* Stage 5: Phosphate Solution Stage 6: Overflowing rinse Stage 7: Sealer Stage 8: Overflowing rinse Stage 9: De-ionised water rinse * the grain refining rinse is not required with iron phosphates. Immersion systems All of the process cycles for spray phosphating can be operated in immersion systems but contact times, temperatures, and concentrations of chemicals will be increased. An important application of immersion methods is in the electrophoretic painting of car bodies. Automatic process control The availability of Programmed Transporter (sometimes called Programmed Hoist ) plants allows considerable flexibility in layout and improved efficiency. This type of equipment is finding more applications as increased volumes of small automotive components are electro-coated. Application by hand spraying The use of programmed hoist systems for pre-paint phosphate applications, especially in conjunction with electro deposition paint applications optimises the use of floor space, capital expense, and production throughput. The system also reduces waste treatment requirements. Programmed hoist plants are used for products such as fasteners, wire, and tubes. Generally immersion times, temperatures, and concentrations are higher in order to produce higher coating weights. In these cases rack type hangers are used for wire and tube bundles, but fasteners will usually be processed in rotating barrels, to simplify handling. This equipment which is widely used for processing small components is often described as Barrel Phosphating Plant. 10

11 Quality Control Quality control is of paramount importance to ensure consistent quality and product performance. Pre-treatment chemistry is a dynamic process and suppliers have developed sophisticated systems to ensure that specifications are met. On-line dosing controls can be automated ensuring all chemical parameters are consistent. Reliable end product test methods have been developed. These include Cross-Hatch or Dry Adhesion testing and corrosion testing by Salt-Spray and Humidity methods. National and International Standards have been introduced to specify the quality control and performance requirements. The result is a complete chemical system meeting all requirements. Chemical suppliers also provide extensive onsite technical support for their customers. Scanning electron microscope (SEM) with EDX analyser for surface examination Specifications covering phosphating and related topics. British Standard Defence Standard Defence Standard Defence Standard British Standard 4842 British Standard 6496 British Standard 6497 British Standard EN Cleaning and preparation of metal surfaces. Phosphate treatment of iron and steel. Chromate conversion coatings for aluminium and aluminium and aluminium alloys. Wet painting of architectural aluminium. Powder coating of architectural aluminium. Powder coating of galvanised steel. Phosphate conversion coating for metals. Method of specifying requirements. Quality control downstream of a spray pickling line Impedance Spectroscope for the electro-chemical examination of conversion coatings 11

12 Reducing Environmental impact Environmental Care Chemical based metal finishing operations generate waste products of environmental concern. Increasing environmental awareness and legislation, reduction in discharge consent limits and the need to control process costs has demanded careful attention to waste minimisation. Also a number of the compounds traditionally used have been re-classified as dangerously hazardous materials. The industry has successfully developed safer alternatives. In the phosphate process the sources of waste are: Cleaners that are dumped when they become contaminated. Big improvements have been made in the development of long life Alkali cleaners. Cleaners based on old technology became saturated and started to redeposit removed oils. Such cleaners were frequently dumped after being used for one week, requiring waste treatment to separate oils, neutralisation and discharge. Today there are oil-rejecting cleaners that will continuously split the oils from the cleaner for removal with oil skimming or coalescing equipment. Cleaners that were previously dumped weekly can now last for three to six months without replacement. Other techniques designed to increase cleaner life include micro-filtration. Descaling acids (pickles) that have reached high levels of dissolved iron. Pickles can be regenerated through the use of ion exchange resins. The most economical are phosphoric acid and citric acid pickles, however even sulphuric and hydrochloric acid are now being regenerated using chilling techniques to precipitate the metals from super saturated solutions. Phosphate solutions at the end of their life. Zinc Phosphate baths contain heavy metals including zinc, manganese, nickel, and iron. In addition, phosphates in effluent streams feed algae growth and must be removed. Processes vary in the degree to which they impact on the environment. Chemical suppliers can advise users on the selection of processes that will minimise effluent problems. Rinses. It is essential to maintain rinses at a specified low contamination level. The greatest volume of effluent to be treated is from overflowing rinses. These outflows contain very low levels of contaminants that are difficult to treat. By using conductivity controls volumes of rinse overflow can be minimised at acceptable levels of contamination. This technique can dramatically reduce water consumption. Another important way to reduce the volume of rinse water is to install counter flowing rinses. By incorporating two rinses, with fresh water entering the second rinse, after each chemical stage the drag out is concentrated in the first rinse, and conductivity control is used to maintain the second rinse at low contaminant levels. Effluent volumes can be reduced by up to 80% through effective design of counter flow rinses and controls. While the inclusion of extra tanks may lengthen the plant, and increase initial cost, the investment can be rapidly recovered through reduced water consumption and treatment costs. Final Seals were traditionally chromium based materials. Whilst chromate waste can be easily treated, the possibility of accidental discharge has created greater awareness of potential hazards, and in many cases, chromate based materials have been replaced with non-chromium materials. These new materials do not require significant treatment, and present minimal environmental hazards. New technology and good planning can allow phosphate systems to be operated with minimal environmental problems. In all cases it is essential to include the potential chemical suppliers in the design stage of the plant. Paint systems present different environmental problems. Today there are excellent alternatives to traditional solvent based materials. 1. Electrodeposition (Electrophoretic) painting, in which the paint is electro deposited onto the surface from water dispersions of paint. These systems virtually eliminate air pollution from evaporating solvents. 2. Powder coatings are free from solvents. 3. When considering conventional applications such as electrostatic spray, or dip and drain, there are many water based paint formulations that offer minimal air pollution potential. Published in the interests of the surface finishing industry by: British Surface Treatment Suppliers Association Federation House, 10 Vyse Street, Birmingham B18 6LT. England. Telephone: /3 Fax: enquiries@bstsa.org.uk web site: The BSTSA is a member of the Surface Engineering Association