Phosphate Conversion Coating Weight by Strip and Weigh

The arithmetic is a division by area, and THE AREA IS BOTH FACES OF A FLAT PANEL. A 4 by 6 in panel is 48 sq in, not 24, and dividing by one face reports EXACTLY DOUBLE -- enough to turn a failing coating into a passing one with a correct measurement, a correct balance and a wrong area. That is the single most common error in the test, so the one-face figure is computed and shown alongside the correct one rather than left as a warning. COATING WEIGHT IS A PROXY AND IT IS WORTH BEING CLEAR ABOUT WHAT FOR. What matters is that the conversion coating covers the surface completely with a crystal structure fine enough to key the paint and to resist undercutting corrosion. None of that is weighable, so the specification asks for a mass per unit area, which correlates with coverage well enough to control a running line and says nothing about crystal size or uniformity on its own. BOTH FAILURE DIRECTIONS ARE REAL AND THEY LOOK DIFFERENT, which is why a specification states a RANGE rather than a minimum. A light coating leaves bare or thinly covered areas where paint adhesion and corrosion resistance both fail, and it shows up as blistering or filiform corrosion in service. A heavy coating is coarse and friable and fails within itself under the paint film -- the paint is well adhered to a layer that is not well adhered to the metal. Zinc phosphate for a paint base commonly runs roughly 150 to 300 mg per sq ft and iron phosphate roughly 30 to 80, and a result that falls between the two bands is the signature of a zinc phosphate bath running short rather than a misidentified process. THE STRIP SOLUTION IS THE PART THAT QUIETLY INVALIDATES RESULTS. It has to dissolve the coating and leave the substrate alone, which makes it specific to both: a solution correct for zinc phosphate on steel will attack a different base metal, and one that takes metal along with the coating reports a coating weight that is partly substrate. A result that seems too high for the process is worth checking against the strip before it is believed. This is a measurement calculation on entered figures. The stripping procedure, solution, time and temperature are prescribed by the applicable test method for the specific coating and substrate, and a result obtained by any other means is not comparable. Specification ranges are entered and are set by the coating supplier and the paint system that follows; the ranges quoted here are typical rather than authoritative. It does not assess what coating weight is a proxy for -- coverage, crystal size and uniformity, and the absence of bare or flash-rusted areas are evaluated by microscopy and by performance testing, and a correct coating weight does not guarantee any of them. It does not address bath control, the cleaning and rinsing stages that decide whether a conversion coating forms at all, the seal or final rinse, or the drying and the window before painting. It does not evaluate paint adhesion or corrosion performance, which are tested directly. The applicable ASTM test method, the coating supplier's specification, and the paint system requirements govern.

coated area = panel length x width / 144, times the number of coated faces; coating weight in mg per sq ft = mass lost on stripping in mg / coated area; g per m^2 = mg per sq ft x 10.7639 / 1,000.

The strip-and-weigh conversion coating weight method. The stripping solution, time and temperature are prescribed by the applicable ASTM test method for the specific coating and substrate; specification ranges are ENTERED from the coating supplier.

A division by area and one unit conversion.

Estimate. AHJ and licensed professional govern.

Field names used by the API: panel_length_in, panel_width_in, faces_coated, mass_before_g, mass_after_g, spec_min_mg_ft2, spec_max_mg_ft2, area_one_face_ft2, area_coated_ft2, mass_lost_mg, coating_mg_ft2, coating_g_m2, one_sided_mg_ft2

• Both faces are coated unless told otherwise the default is 2; the one-face figure is reported either waythe geometry of a flat test panel
• Typical ranges are entered, not assumed zinc phosphate roughly 150 to 300 mg/sq ft, iron phosphate roughly 30 to 80the coating supplier's specification
• The strip removes coating only a solution that takes substrate reports a coating weight that is partly base metalthe applicable ASTM test method