Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 139 of 843

Process Time Sequence Adhesion Creepage From Scribe Normal Time Sequence 5B 0.1 mm 3-Minutes in Stages 5B 0.2 mm 3-Minutes in Drain Zones 5B 2.9 mm 3-Minutes in Stage & Drain Zones 5B 0.4 mm Table 1. Adhesion Test Results iron phosphating. The chance for something unforeseen with TMC is possible. If there is still uncertainty about how the new technology will perform on your line, then conduct a short-term trial. Consider a 5-stage pretreatment system with the traditional alignment of cleaner, rinse, phosphate, rinse, final rinse. With some effort and a garden hose for auxiliary rinsing you can conduct a short-term TMC after a shift or on a weekend by putting the TMC in stage 4. Run for an hour or two or set a number of parts to treat, test and set the rest on the side for eventual use. In other words, try it on for size to see if it fits your operation. This will help reassure that a change over has a chance of success or whether some modifications are required. If you have multiple paint lines then perhaps you can start your testing on the smallest line or the line with the least amount of risk. WHAT CAN GO WRONG Water quality is very important for most if not all TMCs. Cations such as calcium, iron, and magnesium can precipitate vital ingredients of TMCs. If the water hardness isn't too hard then you might not need it. Even if the TMC can tolerate some water hardness, there is concern about its use in the final rinse. The water cations dried on the substrate are hygroscopic and will absorb moisture to form blisters in moist environments, compromising paint adhesion which leads to corrosion. The anions such as sulfate and chloride can initiate corrosion. Poor rinsing and excessive carry over can also create performance issues. The TMC products operate with very low solids in the treatment solution. In other words, they don't handle contamination very well especially alkaline carry over as it precipitates the active metals in the TMC solution. Lines with insufficient rinsing from lack of stages, poor riser/nozzle performance, or short drain zones lead to quality or product usage issues. High chemical usage can also occur if the solution control procedures aren't adequate. Insufficient cleaning causes most finishing issues. Obviously all organic soils need to be removed. Inorganic soil removal is more important for TMCs than for phosphating. Phosphate-based surface conversions are more forgiving for metal variations simply due to the phosphate reaction etching the substrate to start the phosphate conversion. The TMCs do not etch the substrate to this extent to produce a surface conversion. If there is a thin layer of organic or inorganic soil, the phosphate etching will undermine the residual soil to give you a passivation. Inorganic compounds formed on the substrate from aging or from reaction with certain lubricant components can interfere with the continuity of the surface conversion. The inorganic compounds and transitional metal oxides are electro-chemically different. The electro-chemical difference 136

Articles in this issue

view archives of Metal Finishing Guide Book - 2013