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Through product testing, a precision castings manufacturer was able to find a suitable alternative to TCE for cleaning prior to fluorescent penetrant inspection of aerospace components and other parts.
Vapor Degreasing Solvent Property Comparison
Vapor Degreasing Cleaning Process
PROBLEM: The need to replace TCE with a non-hazardous alternative vapor degreasing solvent
SOLUTION: A binary azeotrope of n-propyl bromide and isopropyl alcohol
RESULTS: Better cleaning results in the same vapor degreasing equipment while meeting environmental, health, and safety requirements
A meticulous cleaning process is critical for metal components prior to fluorescent penetrant inspection (FPI). One commonly used cleaning method that is both simple and effective is vapor degreasing with trichloroethylene (TCE). However, TCE is classified as a suspected carcinogen and as a hazardous air pollutant (HAP), raising toxicity concerns and requiring permits (local and state) for its use.
A manufacturer of precision castings employing TCE in a vapor degreasing cleaning process prior to their FPI process began a search for a replacement solvent. The company considered a number of “drop-in” alternatives, but after in-depth testing on actual parts, chose a binary azeotrope of n-propyl bromide and isopropyl alcohol. This solvent, marketed under the name LENIUM ES by Petroferm Inc., proved to provide equivalent or better indication results than TCE. The most dramatic improvement was found in Level 3 (high sensitivity) inspection. This non-hazardous alternative vapor degreasing solvent also is compatible with metallic alloys, is non-flammable, and is non-carcinogenic. It is comprised of SNAP (Significant New Alternative Program) approved materials.
The FPI Process
The fluorescent penetrant process employs a series of process chemicals—penetrant, cleaning solutions to remove penetrant, and developer powders—that are used for inspecting metal parts to detect cracks or other metal imperfections that could cause failures. The process is sometimes referred to as non-destructive testing (NDT), as it allows for inspecting the part using non-invasive methods. Both the aerospace and automotive industries view this particular inspection process as critical for high reliability components, since failures could lead to catastrophic results.
The purpose of the vapor degreasing process prior to FPI is to remove all metalworking fluids (coolants, sludge and oils) and debris embedded in the pores, cracks, or voids within the component that would prevent the penetrant from entering the defect. If the penetrant is blocked from entering the defect area, that area will go undetected, giving a false indication that no defect exists.
Besides TCE vapor degreasing, a number of other cleaning processes have been used, with varying levels of effectiveness, to prepare components for FPI:
- Manual cleaning using petroleum-based solvents such as acetone, methyl ethyl ketone (MEK), toluene and mineral spirits
- Vapor degreasing with hydrochlorofluorocarbons (HCFCs); other chlorinated solvents such as perchloroethylene (PERC) and methylene chloride (MC)
In this case, the precision castings manufacturer was in search of a suitable alternative to TCE for use prior to a fluorescent penetrant inspection process. Criteria for success was based on finding a solvent that would perform as well as, if not better than, TCE; could be used in the same vapor degreasing equipment; and could offer improvements over TCE from an environmental, health, and safety standpoint.
Test Component Preparation
The cleaning process for metal components at this manufacturer employs a typical two-sump vapor degreaser in a standard, three-stage vapor degreasing process. The cleaned components are inspected immediately after degreasing using the FPI process. Upon passing inspection, the components are further processed into the final product.
For this study, a series of four cleaning tests were completed over a six-month period at an off-site location. For each test run, the company evaluated the ability to clean a sample size of at least 200 components. Each test component was produced in-house to best emulate the typical conditions and requirements of the manufacturing process, including saturation in cutting oils and metal fines. Each test component was then stacked and racked in the same baskets that were used in the existing degreasing process. The test components, once cleaned, were then inspected using the current FPI process. All of the test components were subjected to the same pass/fail rating as TCE-cleaned parts.
Cleaning Product Under Evaluation
This study included an extensive investigation of the binary azeotrope of n-propyl bromide and isopropyl alcohol. This product effectively removes oils, greases and particulates. It has compatibility with metallic alloys and some plastics. Table 1 gives a comparison of TCE and the solvent.
Cleaning Process and Equipment
In all cases, the components were subjected to the same vapor degreasing steps: immersion in the boil sump followed by immersion in the rinse sump and suspension in the vapor zone. The description of the cleaning parameters for each test can be found in Table 2. The only significant difference was the higher temperature required for the boiling point of TCE. Each test was performed at an equipment manufacturer’s facility under the direct supervision of representatives from the parts manufacturer and the provider of each solvent being tested.
Test results from the cleaning trials indicated that the Lenium ES is a suitable replacement for TCE in the FPI process. The company has proceeded to incorporate the azeotrope in its cleaning and has since seen more consistent detection of component flaws in Level 3 (high sensitivity) inspection.
Jeffrey J. Beard is the national sales and distribution manager at Petroferm Inc. in Gurnee, Ill. Learn more about the company by calling 847-249-6334/6366, emailing precision@petroferm.com or visiting petroferm.com.
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