N221-013 TITLE: Development of High-Viscosity Pre-Penetrant Etching Materials

OUSD (R&E) MODERNIZATION PRIORITY: General Warfighting Requirements (GWR)

TECHNOLOGY AREA(S): Materials / Processes

OBJECTIVE: Develop high-viscosity pre-penetrant etchant materials for aluminum alloys.

DESCRIPTION: Fluorescent penetrant nondestructive inspection (NDI) processes are utilized to detect surface-breaking cracks and corrosion in aircraft structures. Most military aircraft structures are made of 7000 series aluminum alloys (with 7075/7050/7085 being most common). The parts are primed and painted to provide protection from corrosion. Removal of paint schemes or corrosion is often performed through mechanically abrasive processes such as sanding, grinding, or machining, which smear small amounts of material over fine cracks and corrosion, making them less detectable with the penetrant inspection method. Chemical etching is used throughout the NDI industry as a method to remove approximately 0.0002 in. (0.00508 mm) of smeared metal prior to penetrant inspections [Refs 1 & 2]. The etching process typically requires multiple steps, which include precleaning the area, applying an etchant, applying a neutralizer, and applying a desmutting agent [Refs 1�3]. The etchant, neutralizer, and desmutting agents are typically acidic or alkaline and pose some safety hazards, as well as hazards to the aircraft when used in the field. The low-viscosity chemicals (similar to water) are prone to spilling and migrating into crevices in the structures (faying surfaces and fastener holes) near the inspection zone.

This SBIR topic seeks to develop paste forms of viable etchant materials with viscosities similar to toothpaste (70,000 to 100,000 cP) to reduce the hazards of using these chemicals during inspections of parts while they are still installed in the aircraft.

Various chemicals are currently used for these tasks and may be suitable for viscosity tailoring. The most common chemical mixtures used by NAVAIR are:

• Etchant: 0.705 oz (20 g) sodium hydroxide in 3.38 oz (100 ml) water
• De-smutting agent: 1.69 oz (50 ml) nitric acid added to 1.69 oz (50 ml) water
• Neutralizer: 0.353 oz (10mg) sodium bicarbonate in 3.38 oz (100 ml) water

Alternate combinations of chemicals, or other nonchemical processes that can evenly remove 0.0002 in. (0.00508 mm) of aluminum without smearing, are viable alternate approaches.

Etchant, neutralizer, and de-smut materials should have:

• Viscosity of 70,000 to 100,000 cP
• Shelf life of 6 months minimum (single-use needs for areas up to 16 in.� (40.64cm�))
• Useable temperature range of 50F (10C) to 120F (49C)
• Etch rates of 0.00002�0.0001 in. (0.00508�0.00254 mm)/min (removes 0.0002 in. (0.00508 mm) in 2�10 min)
• Etch rate should be uniform
• Chemicals should not require the user to mix components.

PHASE I: Develop, design, and demonstrate feasibility of etchant, neutralizer, and de-smut materials as described in the Description.

Phase I should include laboratory measurements of the viscosities of the chemicals and tests to demonstrate etch rates. Etch rates and uniformity of etching should be substantiated by laboratory testing and microscopy.

If a nonchemical approach is proposed, Phase I tasking should focus on demonstrating the proper etch rates and uniformity requirements are achieved. If the process poses safety hazards or other potential hazards to the aircraft, those hazards should be assessed and mitigated. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Phase II should focus on refining the proposed solution, demonstrating minimum shelf-life requirements, and optimizing packaging/storage concepts for single-use needs for areas up to 16 in.� (40.64 cm�).

PHASE III DUAL USE APPLICATIONS: Verify, validate, and finalize the prototype. Transition to applicable naval platforms and depots.

Pre-penetrant etch is required before penetrant inspection processes when any mechanical working (sanding, grinding, blasting, machining, etc.) of the part�s metal surface during manufacturing or reworking is done. Penetrant inspections cannot be performed on painted parts, so mechanical paint stripping in-service parts requires etching before penetrant inspection can be performed. Penetrant nondestructive testing (NDT) is commonly used in a multitude of industries including infrastructure (buildings/bridges), transportation (auto/rail/ship), energy (oil and gas/hydrodynamic/wind), and space (rockets/payloads). Users of the penetrant NDT process could benefit from a safer etching process.

REFERENCES:

1. Center for Nondestructive Evaluation. (n.d.). Material smear and its removal. Iowa State University. Retrieved July 1, 2021, from https://www.nde-ed.org/NDETechniques/PenetrantTest/MethodsTech/materialsmear.xhtml.
2. Moore, P. O., & Moore, D. G. (2016). Nondestructive testing handbook: Liquid penetrant testing (Vol. 1). American Society for Nondestructive Testing. https://www.worldcat.org/title/nondestructive-testing-handbook-volume-1-liquid-penetrant-testing/oclc/1039286877&referer=brief_results.
3. Rainone, B., & Statham, A. (2011, October). Pre-penetrant etch. NADCAP NDIT Non-Destructive Testing Newsletter. https://cdn.p-r-i.org/wp-content/uploads/2012/09/11094218/P111548NDTNewsletter.pdf.

KEYWORDS: Penetrant; etchant; metal; smear; nondestructive; non-destructive