Nital Micro Etchants:
Nital Etch Inspection (sometimes referred to as “Temper Etch”) is a non-destructive testing technique used to evaluate the effects of manufacturing processes such as machining, grinding and heat treatment on metallic components. Under certain circumstances machining and grinding can introduce sufficient heat into a component to heat-treat its surface. This is generally caused by local over-heating and can produce undesirable changes in material properties.
Using a Nitric acid solution, machined components are etched revealing grinding and machining abuse. The nital acid etchants work by forming a colored surface film on the steel and attacking its surface to form
The etched surface is then microscopically examined to detect detrimental microstructural modifications (e.g. untempered martensite) that may have resulted.
Such etching is generally performed by immersion in a “Nital” solution containing nitric acid in a carrier solvent of alcohol with the acid concentration ranging from 1% up to about 10%.
The active ingredient in Nital formulations, as the name implies, is nitric acid. The nitric acid concentration commonly ranges from 1 to 4% for most Mil-Specs, depending upon whether it is intended for micro-etch or macro-etch capability.
Due to chemical incompatibilities of ethanol at high nitric concentrations, the safer carrier solvent typically used is methanol.
MIL-S-5002, MIL-STD-867, LAO103-020
Goodrich Actuation Systems (formerly TRW Lucas Aerospace): Specification #981-060-016, LGPS 3000.
Lockheed Martin: NDTS-9111, 5PTPCG10, LMA-PC010
Bombardier Aerospace: MPS 176-14
Boeing Nital Etch Specifications: BAC 5436, DPS4.715, PS21205
Vought Aircraft Industries: 208-9-100, GP 17 G