October 2009 Archives

The word "metal etching" in general may often be associated with metallurgical examinations of alloys where prepared test specimen are chemically or electro-chemically corroded (etched). But this type of etching is only for observations and measurements. Similarly, "etching" of heat treated/machined parts to remove scales and altered layers with various pickling acids, as well as acid/alkaline corrosion embossing have been customarily used, but they are for surface preparations prior to other surface treatment processes.

The "etching" processes we are about to cover in this section are, for instance: copper etching for printed circuit boards where traces are created by dissolving only the prescribed sections of the copper on copper lined plastic substrates; etching of copper alloys and stainless steel alloys to create indelible characters and patterns by corrosion engraving for name plates and printing plates; and etching process used to create patterned holes on thin metal plates. In other words, the "etching" in discussion here is equivalent to machining of parts, and therefore often called "Chemical Milling". The word "etching" is interpreted as "corrosion" in Japanese language, but technical literature defines it as "Etching is a process of chemically removing materials from substrates according to given matching patterns". There are two types, the deep etching and superficial (surface) etching, of etching processes. The former type of etching is used for forming outlines and creating holes. The latter type removes very little material where the etching agent only roughen the surfaces, and typically used for name plates and the like. This section will cover the former type "Deep etching". There are two types of etching processes: chemical etching; and electrolytic etching. The chemical etching uses chemical-resist patterns drawn on metal substrates where the exposed metal is removed by the etchant. The electrolytic etching electrochemically removes materials by using electrodes formed into intended shapes as cathodes and the object itself as an anode. Electrolyte fills the space between the two and electrolysis using a direct current removes the intended material.

There are more hydrogen embrittlement testing methods as follows.

(5) Delayed failure test using the actual part

In order to obtain the final confirmation on whether the plated part is likely to break due to the hydrogen occluded during the plating processes, or the baking process' effectiveness of hydrogen embrittlement removal, a delayed failure test using the actual part must be performed.
This is also necessary for failure analysis when they occur, as well as when there are changes in part designs and intended applications for the part.
Although the problems regarding failures are complex and the delayed failure tests may not always reveal all problems, confirmation tests are still needed for problem analysis.
A good measure in devising the delayed failure tests using the actual part is to consider the individual application specifics and design an appropriate scheme. Failures due to hydrogen embrittlement occur suddenly by the presence of occluded hydrogen, and at a location of notched feature where tensile stresses concentrate. It is best to replicate this condition.
An ideal method is the one adopted by the aircraft industry. If the part has a notched feature, a static load equal to 70% (90% if not notched) of the part's ultimate tensile strength is applied. If the part does not fail this delayed failure test within 200 hours, the part is deemed as passed.

An evaluation method using Delta Gage is introduced below. A plated test specimen is placed in a vise of a hydrogen embrittlement test system, and the vise is closed and the test specimen is flexed to 90~95% of the distance where the test specimen actually failed. The condition is held static and the time it took for the test specimen to fail was measured. There is a clear correlation between the hydrogen embrittlement rate measured by the Delta Gage and failures as results are shown in [Table 1]

[Table 1] Delayed failure test results

There are more hydrogen embrittlement testing methods as follows.

(4) Lawrence Hydrogen Detection Gage

This testing method was developed by a Boeing engineer Mr. Lawrence in cooperation with Mr. Takada for the purposes of plating bath management, and determining the effects and safety of aircraft landing gear cleaning solution and paint remover in relation to hydrogen embrittlement.

This method uses a metal cased vacuum tube as a probe to detect the hydrogen atom intrusion penetrating through the vacuum tube's metal case wall. The metal tube case (probe) is used as the cathode in the plating process. Hydrogen atoms generated during the plating process penetrates the coating layer and the steel case wall, and ionize when intrude into the vacuum tube. Here, an ionization gage is used to monitor and record the presence of hydrogen atoms.

Next, the plated probe is dried and placed into a 200 deg.C oven associated with the test equipment, and a baking process is performed. By the baking process, the hydrogen atoms occluded by the coating layer and the steel case wall diffusively migrates rapidly into the vacuum tube.

A barium getter is placed inside the vacuum tube to absorb and remove the migrated hydrogen at a constant rate. The vacuum level inside the probe can be interpreted as the change in hydrogen release rate. As the result, plating layer's hydrogen release characteristics can be determined.

As seen above, this testing method can measure the amount of hydrogen intrusion during the plating process, and hydrogen release characteristics by baking process.

[Fig.1] is a conceptual diagram of the testing probe. It explains how occluded hydrogen by the plating layer is measured.

1） Electrons (e) generated from the heated cathode are attracted to hydrogen molecules inside the vacuum tube. 2） The electrons accelerate when passing through the positively charged grid, and collide with the hydrogen molecules. H2+e→2H+Hydrogen molecules become ionized. 3） Ionized hydrogen H+ is attracted to #2 cathode and transforms into an electrical current, then read by a meter.

There are other methods to measure hydrogen embrittlement.

(2) Notched tensile test

This method is also called Stress Rupture Test or Sustained Load Test, used by US Air Force, Navy, Boeing, and Lockheed Martin, and is considered to be the most accurate means of testing. This is a delayed fracture test with a tensile testing machine where a notched test specimen made of high tensile strength steel is subjected to a static tensile load that is 75% of ultimate tensile strength. If the specimen does not fail within 200 hours of this static loading condition, it is considered to pass the test.
The test specimen is made of high tensile steel alloy (AISI 4340), annealed and machined, heat hardened, and ground finished. Lastly, a V-shaped notch with a v-valley radius of 0.01 inch is added to this test piece. The US military testing standards additionally calls for this test specimen to be cadmium plated after dry-blasting with 100~180 mesh aluminum oxide media. After an appropriate baking process, this test piece is subjected to a static tensile load 75% of the ultimate tensile strength and left alone. If there is hydrogen embrittlement present in the test piece, it will break at the notch in several ~ 100 hours.

(3) Douglas stress ring test

Also called "Stress ring test". this is a testing method provisioned in Douglas Aircraft company's overhaul manuals. This is a delayed fracture test where a prop-bar is inserted in a ring made of high tensile steel to obtain a static load 90% of the ultimate tensile strength. The test specimen is considered as passed the test if it does not break after 200 hours of this static loading condition.
A heat treated ring 60mm dia., 2.5mm thickness, 25mm width made of high tensile steel 4340 is used. Cadmium plating is applied and baked on this ring. The ring is then deformed into an oval shape in a vise for a prop-bar called Stress Bar to be inserted. The bar statically maintains the oval shape to create a stress equal to 90% of the material's ultimate tensile strength.

Reliability of this testing method is widely argued, and no other aero-space manufacturers or the US Air Force do not recognize this method, but since the method does not require any specialized test equipment, it is considered convenient.' C Ring is shown in Fig.1.