February 2012 Archives

(8) Radioactive tracer test

It is said that by making artificial soiling, mixing in some radioactive isotopes (radioactive material) and measuring with a Geiger counter can yield sensitivity 1000 times of the spray pattern test and 100 times of the atomizer test. Since this test yields very high sensitivity, it is useful in comparing the cleanliness levels of various washing methods and detecting minute amounts of substances otherwise undetectable by other means. However, since the radioactive material is difficult to handle in actual process sites, it is not a generally applicable method.

(9) Rust test

There is a pinhole test for electroplated objects and this test is similar to that. A piece of filter paper soaked with potassium ferricyanide and sodium chloride are made to contact the surface of steel. The test utilizes Prussian blue precipitation caused by ferric salt ions reacting with the indicator on cleaned surface, and the sensitivity is said to be 1/10 of water wetting method.
Also, there is a rust method that uses a water solution containing 2% hydrogen peroxide and 0.008% salt where the solution is dripped on the metal surface and observed.

(10) Plating test

This is a method that uses electro plating. The plating adhesion is evaluated as well as salt water spray and wet box testing can also be used. The cleanliness evaluation by examining the plating adhesion condition utilizes the nature where plating adhesion is especially sensitive to inorganic soils such as oxidation films. Also, by applying high bright plating the evaluation can be performed. Incomplete degreasing before plating will cause loss of gloss and may leave residues and fingerprints that are clearly visible.
For surfaces treated with oil based grinding compounds, residual oils remain on the treated surfaces as thin films. These surfaces must be completely degreased and the method to evaluate the difficultness of this is to use a test sample shown in [Fig.1] and Fuff polishing >> alkali degrease >> bright nickel plate >> then evaluate. The plated surface incompletely degreased will show with lost glossiness with residues and fingerprints clearly visible.

The reason for the various sized holes on the plate is to make the level of degreasing by observing differences due to fuffing heat and buff compound residue adhesion.

(6) Fluorescence Test

This test uses fluorescent dye mixed into the soiling. After cleaning, the remaining soil is exposed to a fluorescent light and visually inspected or photographed. The light reflection strength of oil containing the fluorescent dye is proportion to the amount of the oil. Completely clean surface will show as black for the lack of oil (dye) and soiled surface will show the amount of remaining oil by the reflection amount.
Therefore, this test will not soiling containing no dye and cannot be applied to detect normal soiling. Rather, it is useful for evaluating cleaning solution degradation levels and compatibility of cleaning process.

Generally, a luminographic photography system shown in Fig.1 to evaluate. This equipment can detect remaining oil of 0.004mg/cm2 where it was 0.113mg/sm2 before the wash.

(7) Contact angle test

When 0.05mL of water is dropped on a clean metal surface, the droplet will form almost perfectly circle with a flat wave shaped edge, but the droplet will be small and with a non-wave formed edge if there is a thin film of oil on the surface. The droplet will not be circular if a significant amount of oil soiling exists.
It is said that this method can detect a difference of metal surfaces that are completely clean versus a surface with 0.01% oil dissolved in a solution.

By utilizing this principle, soiling can be quantified by measuring the contact angle ([Fig.2]) with a drop of water on a cleaned surface.

[Table 1] shows the amount of oil in the solution versus the contact angles.

[Table 1] Soiling within solution versus contact angle

(4) Spray pattern test

This test evolved from practices of blowing breath on dirty glass surfaces to determine the cleanliness, and is performed by spraying cold and clean water in fine mists onto the test panel. Recommended spraying distance would be approximately 60cm to avoid water extravasation on the surface. The soiled areas will appear obviously like a map pattern. The compressed air source used for this test must of course be free of oils, water and dust particles. After spraying, the test panel will be tilted to remove the excess water as not to cause any drying. The test requires shorter time for heavily soiled panels and longer time for cleaner panels.

There is an improved version of this test that uses a box with a clear acrylic panel with grid lines forming 100 squares. The test panel already sprayed with water is placed inside of this box and view through the grids, and the soil map is traced with felt marker to quantify the cleanliness as a %. [Fig.1] shows a pattern traced by this method.

(5) Atomizer test

This test is a evolved version of the spray pattern test. The difference from it that the evolved version is performed on dry surfaces and dye is added to the water so the pattern will remain even of the water evaporates. As shown in [Fig.2], distilled water is atomized with approx. 450mmHg air sprayed on the test panel at approx. 60cm distance. It takes 30~40 seconds to see where it is soiled and where it is clean.

After spraying, a heat lamp is used to dry the surface to reveal the dyed areas where water adhered. The atomized water droplets are finer than the conventional spray method, minute soiling can be detected.

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