Thickness of phosphate coatings and thickness measurement

A. Hi Kuldeep. I think the first thing to do is to double check for any possibility of a mistake in where this oversize situation developed because you wouldn't want to make changes based on bad info. But after that, Guillermo_Marrufo tells us that an overactive etch or excessive immersion time in the phosphatization stage may cause this.

Luck & Regards,

A. Hi Bhatiya,

Do you mean the actual thickness of the coating, or the amount of phosphate coating that has been deposited?

The actual thickness is difficult to measure because there is no clearly defined boundary between the coating and the base metal. Remember that the top portion of the metal surface has been converted into the phosphate coating.

The amount of coating can be determined gravimetrically by immersing a phosphated panel into a solution of chromic acid.

A good discussion can be found in Werner Rausch's book, "Phosphating of Metals".

A. Most likely the cause may be an over etch during the pre-cleaning or an over exposure to the phosphating solution of the parts.

A. My thought would be that the interference fit is crushing the phosphate to a fine powder which can work out and also act as a lubricant.

Thanks for the responses. I am glad that I could move a step or two forward with these inputs.

A. Most people will use mechanical measurement of before and after, dividing by two if it includes 2 sides. Test panels are not as reliable as sections of scrap parts. Notice that your "before" measurement has to be after any grit blast or chemical attack. Notice also that for really reliable information, the measurement has to be taken in the same spot--exactly. Notice that with repeated measurement, some of the tops of the phosphate will break off or wear off or polish. For electronic measurement, it requires excellent standards, which most people do not have. I do not think that magnetic will work if this is iron phosphate, but you might be able to prove that it does with the right fudge factors, IE: as a go-no go gauge.
So, you come down to what is the required precision of the measurement. Most people would just use weight loss rather than direct measurement, but this requires a lab with a good balance and haz waste disposal (chrome in the stripping agent).

A. The thickness of various types of phosphate coatings will be of the order of 1 to 50 microns. However, for practical purposes, the coating thickness is usually quantified in terms of weight per unit area (usually as g/sq.m or mg/sq.ft) and commonly referred to as coating weight. The reason for the adoption of coating weight rather than coating thickness as the usual measure of coatings is the difficulty in measuring the latter, compounded by the uneven nature of the substrate and of the coating.

The ratio between coating weight (g/sq.m) and coating thickness (microns) varies between 1.5 and 3.5 for the majority of industrial phosphate coatings. For light and medium weight coatings, 1 micron can be regarded as equivalent to 1.5-2.0 g/Sq.m.

A non-destructive method based on specular reflectance infrared absorption (SRIRA) has been used for the determination of zinc phosphate coating weight. X-ray fluorescence is a nondestructive and accurate technique for measuring the thickness of thin phosphate coatings.

Scanning electron microscopy will be useful to determine grain size and uniformity of the coating, as reported by you.

Dear sir,
Thank you very much for your info,
Also we are already applying coating weight method but our customer has requested coating thickness.
We will try to convince our customer with your info.

Regards,

A. It is possible to measure the thickness of a manganese phosphate coating by SEM, but using its EDAX facility. The system will need calibrating, but the principle is that you start with a very low beam energy and then gradually increase it until the detection peak reaches its maximum value. You need to calibrate the beam penetration distance for your selected atom against the beam voltage. The better peak to use will the manganese, because it is the heavier atom, but you may need to take account of any manganese in the substrate. I have successfully used this technique for measuring very thin layers of platinum and palladium - typical thicknesses of less than 100 nm

A. First, Manganese phosphate is typically a very rough finish so that it can hold oil. That said, it is nearly impossible to get valid numbers.
Second, your standard should always be near the range that you desire to measure. See if you can get another standard that is about 20. 70 is far too big for good results.

A. Good day Michele.
There is some info in letter 51028 "Controlling Manganese Phosphate Coating Thickness".
Regards,

A. Hi Michele,

Phosphate coatings are normally measured in coating weight, not thickness (MIL-DTL-16232 [from DLA] is a good source for how to do the test).

The problem with phosphating is that it is highly crystalline, discontinuous and uneven. This means it is nigh on impossible to get accurate readings with either eddy current or magnetic type thickness testers. There all sorts of influences that makes the method inappropriate including inability to place the probe flat to the surface, crystal edge effects, surface discontinuities, etc.

A. Hi Javed. We appended your inquiry to a thread which may fully or partially answer it for you.

Regards,

A. Hi Mario. We appended your inquiry to a thread which helps answer it. In addition to the explanations provided here by TSN Sankara and others, you may also wish to review letter 17826, "Phosphate Coating Weight Specifications". One good standard is TT-C-490 [from DLA] ; another, which I see referenced but which I don't have myself, is Indian Standard IS: 3618. Good luck.

Regards,

A. Hi David. I haven't used a Gardo®meter, but understand that it functions by measuring the reflectance/absorption ratio. If this proprietary testing device is not working properly for you, you should talk to Chemetall about what might be wrong either in your operating procedures or in the device's operation/calibration.

Brian Terry suggests on this thread that MIL-DTL-16232 [from DLA] offers instruction on how to properly determine actual coating weight, and you may be able to do this strip & weigh method in-house or have a third party do it as verification of whether the results from the lab are accurate. Labs aren't alway right :-)

Luck & Regards,