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Coating thickness testing methods: metallographic method, Coulomb method, X-ray method

May 7, 2024

Coating thickness test to detect the thickness of metal and oxide coatings on the surface of materials.


The general testing methods include:


1. Metallographic method


2. Coulomb method


3. X-ray method


Applicable scope


Metallographic method:


The method of using a metallographic microscope to examine the cross-section and measure the local thickness of the metal coating and oxide film layer. Generally, thickness testing needs to be greater than 1um to ensure that the measurement results are within the error range; The thicker the thickness, the smaller the error.


Coulomb method:


Suitable for measuring the thickness of single-layer and multi-layer metal coatings using anodic dissolution Coulomb method, including measuring the thickness of multi-layer systems such as Cu/Ni/Cr, as well as alloy coatings and alloying diffusion layers. Not only can it measure the thickness of the cover layer on flat specimens, but it can also measure the thickness of the cover layer on cylindrical and wire materials, especially suitable for measuring the metal and its potential difference of multi-layer nickel coatings. The types of coatings measured are Au, Ag, Zn, Cu, Ni, dNi, Cr.


X-ray method:


Suitable for measuring the thickness of metal coatings that need to be analyzed in industries such as electroplating and electronic circuit boards. Including: thickness of metal elements such as gold (Au), silver (Ag), tin (Sn), copper (Cu), nickel (Ni), chromium (Cr), etc.


This measurement method can simultaneously measure the thickness and composition of a three-layer covering layer system or a three-layer component.


Testing Principles


Metallographic method:


Using the principle of metallographic microscope, magnify the thickness of the coating for accurate observation and measurement.


Coulomb method:


Using an appropriate electrolyte to anodic dissolve the precisely defined area of the covering layer, a sharp change in the voltage of the electrolytic cell indicates that the covering layer is essentially completely dissolved, and the thickness of the covering layer is calculated based on the amount of electricity consumed. Due to different methods of anodic dissolution, the amount of electricity consumed for measuring the thickness of the covering layer also varies. When dissolving with a constant current density, the time from the start to the end of the test can be calculated; When dissolving with a non constant current density, the accumulated power consumption is calculated and displayed on the electricity meter.


X-ray method:


The X-ray spectroscopy method for determining the thickness of the coating layer is based on the interaction between a strong and narrow multi-color X-ray beam and the substrate and the coating layer. This interaction generates secondary radiation with discrete wavelengths and energy, which has the characteristics of forming a covering layer and matrix elements. There is a certain relationship between the mass per unit area of the covering layer (if the density is known, it is the linear thickness of the covering layer) and the intensity of secondary radiation. This relationship is first determined by the calibration standard block calibration of the coverage layer with known unit area mass. If the density of the covering layer material is known and the actual density is given, then such a standard block can provide a linear thickness of the covering layer.


Sample requirements


Metallographic method:


Due to the fact that the thickness of the sample measured by metallographic method is local, for some samples with inconsistent thickness, the customer needs to specify specific parts. If there are no special requirements, we will take a relatively uniform area for measurement on our own.


Coulomb method:


At present, we can only measure the coating thickness on a flat surface, and the sample requires at least one 5 mm2 flat surface.


X-ray method:


Its area should be at least greater than 0.05 x 0.25mm


Attachment: Quality Evaluation Standards for Electroplated Coatings


The quality inspection of electroplating coatings includes the appearance, thickness, adhesion with the substrate metal, ductility, microhardness, brittleness, corrosion resistance, wear resistance, weldability, etc. Although the specific quality inspection content varies depending on the part and coating, the appearance, thickness, corrosion resistance, and adhesion with the base metal of the coating are the contents that must be inspected for all coatings. The quality inspection methods and evaluation of electroplated coatings, each country has its own national standards and unified international standards; Different enterprises also have corresponding enterprise standards.


1) Appearance of coating


The appearance of the coating is one of the items that must be inspected for any part or plating type. Usually, the appearance of the coating is directly observed with the naked eye under natural light. Its content includes the macroscopic uniformity, color, brightness, crystallization condition, and macroscopic adhesion of the forged layer. Generally speaking, in addition to its unique color and luster, the coating should also be uniform, meticulous, and have good adhesion. It is not allowed to have pinholes, stripes, bubbles, peeling, burrs, nodules, pitting, burning, cracking, peeling, peeling, abnormal color or leakage. But for plated parts, minor defects are generally allowed at the hook.


2) The thickness of the coating


To ensure the performance of the parts, the coating on the surface of the parts must reach a certain thickness. The commonly used methods for measuring coating thickness include two categories: destructive and non-destructive. Destructive thickness measurement methods include anodic dissolution Coulomb method, metallographic method, dissolution weighing method, liquid flow method, droplet method, etc; Non destructive thickness measurement methods include mechanical gauge method, magnetic method, eddy current method β X-ray backscatter method, X-ray spectroscopy, etc. When using these methods to measure the thickness of the coating, please refer to the corresponding national standards.


3) Corrosion resistance of coatings


There are two main types of testing methods for evaluating the corrosion resistance of coatings: natural environment testing and artificial accelerated corrosion testing. The former includes on-site tests under different environmental conditions and atmospheric exposure tests under different climatic conditions. These methods can truly evaluate the corrosion resistance of coatings, but the disadvantage is that the cycle is too long; The latter includes neutral salt spray test (nss), copper acetate accelerated test (cass), corrosion paste test, electrolytic corrosion test, industrial gas corrosion test, wet heat test, etc. All these corrosion resistance tests have corresponding national standards that specify the conditions and evaluation methods for the tests.


4) Adhesion of coating


There are many methods for testing the bonding strength (also known as bonding strength) between electroplating coatings and substrate metals, but they are all qualitative tests. The common testing methods for coating adhesion include friction polishing test, peel test, file test, grid marking test, bending test, thermal vibration test, deep drawing test, etc. Different methods are applicable to different coatings, and there are also different evaluation standards. Please refer to the corresponding national standards for specific use.