Phosphate conversion coatings are applied to metals both as a finish and as an intermediate layer for subsequent coatings.
They serve to:
- provide corrosion resistance
- improve the adhesion of paints and other organic finishes
- facilitate cold forming operations such as drawing and extrusion
- modify the surface friction characteristics to facilitate sliding
At first glance, zinc or manganese phosphate layers may appear equivalent when considered as non-metallic crystalline separating layers, but there are significant differences between them.
ZINC PHOSPHATING primarily aims to improve the corrosion resistance of the base metal and also serves as an excellent pre-painting base, enhancing adhesion for paints and similar coatings.
Since it does not cause significant hydrogen embrittlement, it is widely used to facilitate cold forming of iron and steel. In wire and tube drawing, deep drawing, and cold extrusion processes, zinc phosphating is an essential step in the production cycle. The current levels and speeds of deformation would be difficult to achieve without prior phosphating treatment.
Phosphate Conversion Coatings are produced by immersion in baths containing the appropriate solutions, with continuous agitation of the bath itself.
During processing, the bath becomes enriched with iron, resulting in the deposition of a mixed iron/zinc layer on the surface.
The presence of iron, in addition to influencing the chemical conversion of the phosphate layer, also affects the initial rate of attack on the base metal and the nucleation of the crystals. The chemical attack on the base metal, necessary to form the phosphate conversion layer, mainly occurs on the micro-roughnesses produced by mechanical processing, so that the material simultaneously acquires a coating and an improved surface finish.
PHOSPHATE CONVERSION TREATMENTS alone provide only short-term corrosion protection. To achieve more effective protection, appropriate additional treatments are required depending on the intended use of the phosphated surface, such as the application of protective oils or waxes. The phosphate layer obtained on the surface is firmly anchored to the base metal and, due to its absorbent properties, becomes impregnated with lubricant and retains it tenaciously. Such post-treatments should preferably be carried out on phosphated components immediately after treatment to prevent the onset of corrosion.
THICKNESS OF THE PHOSPHATE LAYER
It is not possible to establish in advance a minimum or maximum limit for the thickness and weight of the phosphate layer, as these vary depending on:
- the material of the part and its surface condition
- previous mechanical and chemical treatments
- the operating conditions of the phosphating process
The company standard specifies a thickness not less than 3 microns and not greater than 6 microns, unless otherwise required by the customer.
In this case, the tolerance for the specified thickness is ±2 microns.
THE THICKNESS IS DETERMINED BY THE MAGNETIC METHOD ACCORDING TO UNI ISO 2178.
When designing the part, it must be considered that the phosphate deposit forms partly at the expense of the base metal and that, for a fraction varying between ½ and ⅔ of the layer thickness, it protrudes beyond the original dimensions of the part.
APPEARANCE OF THE PHOSPHATE LAYER
The phosphate layer should have a uniformly velvety appearance, with color ranging from gray to black.
Minor color variations within a single part, within the above range, are not grounds for rejection.
The layer must have a microcrystalline prismatic structure with rounded edges, compact, uniform, and free from stains, uncoated areas, scratches, dust, or powdery residues.
Minor variations in the appearance of the phosphate coating, due to surface irregularities of the base metal or contact with supports during the phosphating process, are common and are not usually indicative of significant quality variations. Phosphate layers composed of large crystals surrounded by much smaller ones must be excluded. After removal of the oil film, the phosphate layer must not show yellow-ochre dust (indicating sludge in the phosphating bath).
CONTINUITY OF THE PHOSPHATE LAYER
By their nature, phosphate layers are porous; however, excessive porosity may compromise the effectiveness of the coating. The continuity of the coating can be verified through salt spray tests in accordance with UNI ISO 9227.