When paint, coatings, oxide scale, oil or adhesive residues, corrosion layers, carbon deposits, or other contaminants are present on the surface of a component, and these substances remain on the part during penetrant testing—combined with improper procedures at any stage of the test—they can lead to missed defects or false indications, resulting in quality-related incidents.

Drawing on many years of experience in penetrant testing and on several issues identified during instructional support, the author has compiled this article for reference only.

1. Principles, Advantages, and Disadvantages of Penetrant Testing

Penetrant testing is based on the principle of capillary penetration and adsorption in physics: a penetrant is applied to the surface of the test piece, where, under capillary action, it seeps into surface-opening defects. Excess penetrant on the part’s surface is then removed, and an appropriate developer is applied; subsequently, the surface-opening defects are visualized under black light or white light, thereby revealing the defects. This method is commonly referred to as PT.

The advantages of penetrant testing are: simple equipment, relatively easy operation, intuitive defect visualization, straightforward interpretation, and no restrictions on material type. It can be used to inspect various steels as well as non-ferromagnetic materials such as austenitic stainless steel, copper, and aluminum.

The drawbacks are that penetrant testing cannot be used to inspect porous materials, can only detect surface-opening defects on the workpiece, involves the use of materials that are somewhat toxic, and places specific requirements on the surface condition and cleanliness of the test piece; consequently, its application scope is somewhat limited.

2. Causes and Consequences of Missed Detection and Misdiagnosis

Penetrant testing primarily consists of nine major steps: pre-cleaning, drying, penetration, preliminary water washing, emulsification, final water washing, drying, development, and inspection. Among these, pre-cleaning, emulsification, final water washing, and inspection are the most critical; each of these will be discussed in turn below.

2.1 Poor surface condition of parts or inadequate cleaning

a. Paint layer, anodized layer, or oxide scale:

If the paint coating, anodized layer, or oxide scale on the part’s surface is not completely removed, penetrant will penetrate these residual coatings and layers, resulting in false indications that can obscure the background under blacklight. This makes it impossible to distinguish between the presence and absence of defects, leading to missed inspections or false conclusions.

The paint coating is visually visible, whereas the anodized layer is virtually invisible to the naked eye, as shown in Figures 1 and 2.

We expect the surface finish of the part to be as shown in Figure 3.

Figure 1: The wheel hub surface still has residual anodized and paint layers.

(Figure 2: After penetration/developer application on a wheel hub with an anodized layer and a paint coating)

(Figure 3: Normal wheel hub surface condition under blacklight)

b. Oil stains or adhesives:

c. Failure to thoroughly remove oil or adhesive contaminants from the part surface; when such contaminants adsorb the penetrant, they will fluoresce under a blacklight, producing indications that resemble cracks and potentially leading to missed defects or false positives.

For example, P/N: DNS1000-8-26, main wheel attachment bolt for the Boeing 757 aircraft.

(Figure 4: Surface with uncleaned adhesive residue)

(Figure 5: After imaging of the adhesive-coated area)

Its surface is coated with a resinous substance that is extremely difficult to remove. Although the part may appear clean on the surface, once penetrant testing is performed, any residual penetrant trapped in the uncleaned areas—such as the root of the thread and the “R” zone where the bolt shank meets the bolt head—will be very hard to completely remove from the part’s surface. After development, excess penetrant remains on the bolt, making it difficult for inspectors to accurately assess the condition and increasing the risk of missed or false indications.

C. Carbon deposits or corrosion:

Failure to thoroughly remove corrosion layers or carbon deposits from the part surface can result in false indications or spurious defects, leading to missed inspections or misjudgments.

For example: high-temperature components of aircraft engines.

(Figure 6: Extensive, unrecovered carbon deposits on the surface of the APU second-stage stator.)

(Figure 7: After secondary stator penetration in the APU, a large amount of penetrant indication is observed on the carbon deposit layer.)

d. Other attachments:

Fiber from the wiping cloth adhering to the part surface can create false indications, leading to misinterpretation.

2.2 Over-washing or under-washing during removal of excess penetrant

a. Pre-cleaning:

As the name suggests, this refers to over-washing, where, in the process of removing excess penetrant from the part’s surface, the penetrant within the defects is also washed away.

This is typically caused by excessive emulsification time, an overly steep spray angle, or prolonged water rinsing.

After the part is developed, if there is no penetrant within the defect, the defect cannot be revealed, resulting in a missed inspection.

(Figure 8: Visibly apparent cracks on the brake thrust disc of an A319)

(Figure 9: Due to excessive cleaning at the crack, the crack is not clearly visible under blacklight.)

b. Failure to clean:

During the removal of excess penetrant from the part surface, operators, out of concern that the part might be over-washed, fail to thoroughly remove the excess penetrant, resulting in residual penetrant on the surface after development and the appearance of false indications. This makes it difficult to determine whether the part actually contains defects during inspection.

(Figure 10: Insufficient cleaning; the crack area is heavily covered with penetrant.)

(Figure 11: After wiping with a cotton ball dipped in alcohol or acetone, the cracks become very clearly visible.)

2.3 Insufficient blacklight intensity or excessive ambient light

a. Insufficient blacklight intensity:

When a blacklight is used to illuminate penetrant, it emits yellow-green light, to which the human eye is most sensitive. For small defects, only a very small amount of penetrant will be drawn back out from within the defect. If the blacklight’s intensity is insufficient, the penetrant will not fluoresce brightly, making it difficult to detect even minor defects.

b. Ambient light is too bright:

Under blacklight illumination, when the ambient light level exceeds 20 lux, the yellow-green fluorescence emitted by the penetrant is masked by the white light, making it difficult to detect defects in the part.

2.4 Incorrect Operational Techniques Used by Inspectors When Determining Defects

During the inspection process, when a suspected defect is identified, inspectors typically use a cotton swab dipped in alcohol or acetone to gently wipe the suspect area of the part for further evaluation. However, if the swab contains excessive alcohol or acetone, the penetrant within the defect will diffuse too rapidly, preventing the defect from being properly revealed.
In addition, if wiping is not performed in a single direction but rather with repeated back-and-forth motions, minor defects may be obscured. Furthermore, contamination of the part surface with alcohol or acetone can lead to missed inspections or false judgments.

3. Methods for Solving Problems

(1) Poor surface condition of the part or inadequate cleaning:

This can be avoided by ensuring proper pre-treatment: remove the paint layer using paint stripper or sandblasting (where technically feasible).
Use a cleaning agent to remove oil and adhesive residues from the part surfaces; where technically feasible, sandblasting or brushing with a copper wire brush may be employed to remove corrosion layers or carbon deposits, ensuring thorough removal of all other surface contaminants.

If there are areas where the coating cannot be removed, use an eddy current tester to inspect these areas.

(2) Over-washing or under-washing during removal of excess penetrant:

Cleaning is primarily performed on thinner components where the cracks are through-cracks, such as the exhaust edge of engine blades and the edges of A319 brake thrust discs. When removing excess penetrant, because penetrant in other areas is more difficult to remove, the cleaning time must be extended, which can result in the penetrant within the through-cracks at the edges being washed away.

For thin-walled components such as the brake thrust disc of the A319, it is advisable to perform a visual inspection under white light using a 5× magnifying glass after penetrant testing, with particular attention paid to holes, openings, and edges where abrupt changes in geometry occur. If necessary, a eddy-current inspection should be conducted as a follow-up scan to ensure that no defects are missed.

Over-washing is most commonly observed among new trainees or personnel with limited experience in penetrant testing, as they lack practical experience and have not yet mastered the proper standards for removing excess penetrant from part surfaces. Out of concern that excessive washing might lead to missed defects, they tend to err on the side of over-washing, as illustrated in Figures 8 and 9.

When parts are undercleaned, it is difficult for inspectors to make an accurate assessment. There are two approaches to address overcleaning or undercleaning: first, re-clean the parts (or perform another penetration test), followed by drying, development, and inspection. For small areas of undercleaning, a cotton swab dipped in alcohol or acetone can be used to wipe away the penetrant from the part; this must be done under a blacklight.

The other approach is to use a standard test block as the part and follow the penetrant testing procedure step by step, paying particular attention to the cleaning process. With sufficient practice and careful observation, one can master the proper cleaning standards.

(3) Insufficient blacklight intensity or excessive ambient light:

Perform quality control for each shift in accordance with the requirements of the penetrant testing procedure, promptly replace nonconforming blacklight bulbs, and maintain appropriate ambient light levels.

(4) Incorrect operational techniques employed by the inspector when determining defects:

For this particular item, as long as the preceding steps have been properly executed, any defects on the part will be readily apparent.

However, when indications suggestive of defects are observed on a part, it is essential to verify them by gently wiping the suspected areas with a cotton swab moistened with alcohol or acetone. This step is mandatory, and the key is to ensure the following two points:

First, the cotton balls used for wiping should not be too wet; for those dipped in alcohol or acetone, they are ready when, after being pinched between two fingers, no liquid drips.

Second, when wiping, use a blacklight to examine suspicious areas and wipe in a single direction; observe immediately after wiping, and avoid rubbing back and forth haphazardly.

Summary

By summarizing the analysis of the causes of missed inspections and false judgments, as well as the corresponding problem-solving methods, we have gained a clear understanding of the causes and consequences of various missed inspections and false judgments in penetrant testing.

In fact, these problems are entirely solvable—or, rather, entirely avoidable. For beginners or those with limited practical experience, the key is for practitioners to maintain tight control over every potential point of failure and rigorously verify the critical checkpoints at each stage; in this way, errors such as omissions and misjudgments can be effectively prevented.

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