Scale, Oxides, And Corrosion Products

T.O. 33B-1-1 2-22 reducing the amount of penetrant in the void. When on the part surface, they interfere with the forces or mechanism causing penetrant entry and exit from discontinuities. When dry, they tend to absorb moisture that also interferes with penetrant entry and exit. As surface contaminants, they retain the penetrant, leading to a residual background and false indications during inspection. 2.3.4.4.4 Scale, Oxides, And Corrosion Products. a. Scale and oxides generally occur as a result of exposure to high temperatures. Scale and oxides are usually very difficult to remove and may require extreme cleaning methods, such as acid pickling, abrasive blasting or other metal removal operations. Some of these processes can have an adverse effect on the penetrant inspection process and should be avoided, (see paragraph 2.3.6). Corrosion products, particularly from stress corrosion, often occur or are lodged within discontinuities resulting in removal problems. b. Scale, oxides and corrosion products can bridge or partially fill discontinuities restricting penetrant entry. When on the part surface, they interfere with the mechanism of penetration, impeding both penetrant entry and exit from discontinuities. They also retain penetrant on the surface, leading to a high residual background and false indications. Stress corrosion products occur within the flaws and may be impossible to completely remove. Penetrant inspection for stress corrosion cracking flaws generally requires extended dwell times to permit penetrant entry. 2.3.4.4.5 Paint and Other Similar Coatings. Paint and similar coatings are not foreign soils since they are intentionally applied to the part surface as a smooth, continuous layer. However, they can have several adverse effects on the penetrant inspection process. Many of the coatings are elastic and do not form openings when the base metal cracks from service stress. When this occurs, the surface opening is bridged or covered, preventing penetrant entry. Paint coatings can interfere with penetrant inspection process even when they form open cracks along with the base metal. The paint surface texture is much different than bare metal, and it can interfere with the mechanisms causing the penetrant to enter and exit the discontinuity. Paint coatings, especially when they are oxidized, weather checked or cracked, can retain penetrant during removal causing a high residual background or false indications. 2.3.4.4.6 Water or Moisture. Water or moisture on a part can occur from many sources. The most common source is the cleaning process followed by inadequate drying of the part. Water or moisture on the part surface or in the discontinuity seriously interferes with the penetration process. It is essential that water be removed not only from the part surface but also from the inside of any discontinuities that may be present. Moisture in the form of condensation from high humidity or low temperatures may occur and must be removed. An example of condensation occurrence is the spot cleaning of a local area with a volatile solvent. Rapid evaporation of the solvent may cause cooling to a temperature at which condensation occurs (dew point). 2.3.4.4.7 Residues from A Cleaning Process. The chemicals used for cleaning solutions may contain strong alkalis and acids. If not completely removed from the part surface before penetrant inspection, they can interfere with the penetrant process in several ways. They can impede surface wetting and prevent the penetrant from evenly coating the inspection area. They also interfere with the mechanism causing the penetrant to enter and exit discontinuities. Strong alkalis and acids can decompose or degrade dyes and other chemicals in the penetrant, causing weak or faint indications. Chromate residues absorb black light, leaving less energy to excite the fluorescent dyes in the penetrant. 2.3.4.4.8 Residues from Previous Inspection. Residues from penetrant inspection can affect subsequent inspection results and the serviceability of the part. The effects of residues from previous penetrant inspections are discussed in the following paragraphs. The effects on serviceability are discussed in paragraph 2.3.8.1