University of Ulster, Jordanstown, Co. Antrim, UK
Abstract
Hot rolled steel (HRS) is extensively used in a wide range of applications by many different industries such as automotive, domestic appliances, defence etc. It is common knowledge that hot rolled steel comes with oxide scale, often called mill scale, on the surface, due to the hot rolling process. Despite the disadvantage of oxide scale on HRS, it is still one of the most popular materials used in industry due to its availability, cost and ease of profiling properties. One of the most important coating applications for HRS is powder coating, which has a number of advantages over its favourability to wet coating, therefore it is widely used for HRS components in industry, prior to powder coating, to increase corrosion and blister resistance and enhance adhesion pre-treatment systems are used. Pre-treatment systems usually contain five or more stages: cleaning, rinsing, conversion coating, rinsing and passivation. Conversion coating is the most important stage in the pre-treatment process and it is usually phosphating. Phosphating offers many advantages, however it is considered as a hazardous material to human health and the environment. The phosphating process creates sludge, which results in pipe and pump blockages and sludge built up in the phosphating tank.
These concerns have driven chemical companies to conduct research aimed at finding a conversion coating that meets the requirements of health and safety and is environmentally friendly. Some companies have already developed environmentally friendly conversion coating systems which are promoted as ecological material and an alternative to the phosphating process.
The main objective of this chapter is to evaluate the ability of commercially available environmentally friendly pre-treatment systems as a metal pre-treatment in finishing operations, to eliminate or reduce the amount of environmentally hazardous and toxic chemicals. This objective must be accomplished whilst maintaining equal or better product performance properties, with economic benefit or no significant economic penalty to the metal finishing companies who would like to change their pre-treatment system to an environmentally friendly pre-treatment system. The evaluation focuses on technical performance and economics while validating the laboratory tests and environmental benefits.
In order to evaluate the conversion coatings’ performance studies on: corrosion behaviour, adhesion and blister resistance, salt spray, prohesion test, Electrochemical Impedance Spectroscopy (EIS) measurement, cross hatch test, conical bend test, pull-off test, humidity test and surface morphology were performed. In this chapter the most popular environmentally friendly conversion coatings were evaluated. Environmentally friendly coatings are usually Silane and Zirconium based.
Introduction
Chemical conversion coatings containing phosphate are widely used in the engineering industry for pre-treatment of aluminium, alloys and steel [4.[1] ]. Phosphate conversion coating is known to furnish a moderate degree of corrosion protection to the unpainted substrates and provides an excellent base for adhesion of organic coatings [4.[2] ].
Phosphating offers many advantages; however it is considered as a hazardous material and is costly to operate, as it is uneconomical to process in waste water treatment because it has a high concentration of metal ions and acid and an extremely reactive treating agent. Treatment with a phosphating agent results in water-insoluble salts, which are deposited as a precipitate.
Such a precipitate is generally referred to as sludge and considered problematic due to the cost of removal. Sludge creates many issues such as:
• Creation of high volumes of waste.
• Clogging of spray nozzles: Sludge blocks spray nozzles in the spray cabinet, reducing contact between conversion solution and substrate. This creates quality and maintenance issues such as weekly acid cleaning of spray nozzles.
• Scaling on the inside of the washer: Sludge built-up in the spray cabinets, requires acidic cleaning, which introduces health and safety related issues.
• Blocking of pipes and pumps. Blocked pipes and pumps affect the quality and performance of the conversion coating and require weekly maintenance therefore, both cost and health and safety related issues increase. Unexpected pump failure due to sludge is a common problem in phosphating plants. Figure 1 shows spray pipes and pumps blocked by sludge.
• High operating temperature: Sludge inside the tank covers the heat exchanger preventing transfer of heat to the tank solution. Therefore heating energy cost for conversion coating solution increases. Figure 2 shows sludge built-up in the zinc phosphate tank.
• Excessive sludge built-up in the phosphating tank: Removal of phosphate sludge build-up from the tank is required in addition to the scheduled “acid cleans” normally performed during plant shut down. Health and safety risks are associated with this operation i.e. handling of dangerous chemicals for operators and contractors. There is an additional cost for the handling and removal of special waste as well as the cost of acid solution. There is also risk associated with effluent treatment plant having to cope with additional rinse water from phosphate tank. Figure 2 shows before and after cleaning of phosphate tank.
Figure 1. Blocked Spray Pipe and Pump.
Figure 2. Zinc Phosphate Tank a) Before Cleaning the Sludge Built-up b) After Cleaning.
Since a phosphate ion may burden the environment by eutrophication, it takes efforts to treat liquid waste and therefore it is preferable not to use it [4.[3] ]. Zinc phosphating sludge, in general, has 20 wt.% iron, 10 wt.% zinc, 1-3 wt.% manganese, <1 wt.% nickel and 50-55 wt.% phosphate (composition on dry basis) [4.[4] ]. As a consequence, there is a risk that the use of such coatings will be restricted in the future [4.[5] ]. Furthermore, there is the occupational health and safety issues arising from the risk to workers exposed to these chemicals, the cost and potential liabilities resulting from accidental leakage into the environment and waste disposal issues from normal finishing operations. These make the use of phosphating conversion coatings unattractive to the metal finishing industry. Phosphating conversion coatings are currently subject to regulation under the Clean Air Act, Clean Water Act, and the Resource Conservation and Recovery Act. Control of Substances Hazardous to Health (COSHH) Regulations require a risk assessment for any substance to determine the potential hazard, and other regulations relate to water discharge and its impact on watercourses. Legislation includes Water Order and Water Act, which restrict discharge of certain substances into watercourses.
Energy cost related to phosphating is another burden issue for industries. Most of the zinc phosphating baths in current use require high temperatures and long treatment times, which is a luxury for energy conscious industries [4.[6] ]. The traditional phosphating tank works in a temperature range between 45 °C to 60 °C. The phosphating tanks are usually heated with gas
a b
burners. In addition the requirement for a passivation stage on the phosphating process is unfavourable because of the cost of chemicals, energy, and maintenance. To allay these concerns, many surface finishing chemical companies are attempting to qualify an environmentally friendly conversion coating that meets the requirements of engineering companies.
These products may be named by specific trade identification or by general descriptions such as nano-technology, silane technology or phosphorus-free pre-treatment. Most of these developing technologies use similar key components for creating a surface conversion.
Fluoro-based chemistries operate in low or ambient temperature, but they also generate a small amount of sludge during production of a corrosion resistant conversion layer, which is promoted as improved technology over conventional phosphating. However all these newly developed pre-treatment systems have not been widely used due to the systems being new and unproven. This is despite the fact that they may be able to meet all of the specifications that phosphating conversion coatings offer.
The overall objective of this section was to evaluate the ability of commercially and environmentally available pre-treatment systems, as a metal pre-treatment in finishing operations, which can be used to eliminate or reduce the amount of environmentally hazardous and toxic chemicals. This objective must be accomplished while maintaining equal or better product performance properties, with economic benefit or no significant economic penalty to the surface finishing companies seeking to change their pre-treatment system to an environmentally friendly pre-treatment system. The evaluation focused on technical performance and economics while validating the laboratory tests and environmental benefits.
In order to evaluate the conversion coatings’ performance tests such as: corrosion behaviour, adhesion and blister resistance, Salt spray, prohesion test, Electrochemical Impedance Spectroscopy (EIS) measurement, cross hatch test, conical bend test, pull-off test, humidity test and surface morphology study were performed.