We have discussed various cleaning methods. From this Vol., cleanliness testing methods will be discussed. There are, unfortunately, no definitive method for testing for cleanliness of metal surfaces cleaned by the various methods currently. This is so since the cleanliness desired will depend on the downstream surface treatment process awaiting after the cleaning process.

In actual process work sites, experience based tests such as water drop tests and water draining tests. These tests are used since they are simple, easy, and can be performed in short amounts of time as they are quite pragmatic for the work sites. There are various cleanliness tests being performed in various process scenes as shown below.

(1) Visual inspections

Visually compare the specimen against prepared reference sample for surface stains and oxidation film.

(2) Wiping tests

Metal surfaces are wiped off with tissue paper, filter paper, or white cloth and check the soiling substance wiped off. Although it's a simple method, this is effective in detecting some minute particles missed by other tests. In comparing wet and dry surfaces, it is more likely to detect the minute particles on wet surfaces. However, the wiping pressure applied is important where relatively high pressure will be required to detect the substance called "smut' that may exist on cold rolled steel after an acid pickling process.

(3) Water wetting test

This test utilizes the hydrophilic surface nature where the surface will retain water when no oil exists. Since it only involves wetting the sample with water, it is a simple and non-destructive test used widely. Notes on this test are: (1) water used is to be clean and cold (warm water should not be used since evaporates quickly and promotes rusting), (2) The sensitivity of this test will depend on the water film thickness. Thinner the film, the higher the test sensitivity.

Detail explanation is given below.

4) Spray pattern test
5) Atomizer test
6) Fluorescence test
7) Contact angle test
8) Radioactive tracer test
9) Rusting test
10) Plating test
11) Wear coefficient measurement method
12) Contact electrical differential test
13) Others