Frederick F. Hunt
Cleveland Black Oxide
Edited by Kenneth Korane
Fastener manufacturers routinely apply a protective finish to bolts and screws that prevents rust and corrosion. However, there is some concern that plating processes can contribute to hydrogen contamination in fasteners, resulting in a potential for embrittlement.
Materials undergo hydrogen embrittlement when atomic hydrogen is absorbed and diffused throughout the metal during electroplating or coating. Accepted theories of hydrogen behavior in high-strength materials stipulate that hydrogen is a mobile atom in the crystalline matrix of metals and will, over time, migrate to points of highest stress. This can eventually result in brittle failure.
Generally, higher applied stress means shorter time to failure. And susceptibility to embrittlement tends to increase with the tensile strength or hardness of the steel. Thus, the very design of high-strength fasteners, with their many stress risers and notches and typical exposure to high dynamic loads, makes them ideal candidates for failure by hydrogen embrittlement.
Hydrogen is an acknowledged by-product of many surface-treatment processes including black-oxide coating, and zinc and cadmium plating. Acid cleaning prior to plating is of major concern because it can generate hydrogen on the fastener surface. Plating can involve electrodeposition of protective materials, which can produce hydrogen. The processes also expose fasteners to various cleaning solutions and coatings, many of which have not been fully evaluated with respect to hydrogen contamination.
One reason for a lack of information is that there has been a limited ability to measure hydrogen embrittlement. Recently, a method has been developed to measure the hydrogen-generating potential of coating and plating processes. This test gives a clean bill of health to fasteners with a black-oxide coating.
Black oxiding is a dip process in which parts travel sequentially through a series of tanks. At Cleveland Black Oxide, key steps begin by submersing fasteners in alkaline soap at 180 to 200°F to clean off oil and surface contamination. Parts also travel to an acid pickle tank containing 30% HCl at room temperature. And a caustic solution at 280 to 285?F converts the part surface from Fe2O3 to Fe3O4 and produces the trademark black-oxide finish. Alkaline or water rinses follow each step, and parts typically receive a post treatment of water-soluble or waterdisplacing oil.
ASTM F1624 describes the test to evaluate the effect of hydrogen embrittlement on the properties of fasteners. Testing relies on an incremental step-loading technique, in this case the Rising Test Load method developed by Fracture Diagnostics Inc.
ASTM F519 and F1940 document a method for evaluating plating and coating processes for high-performance fasteners using a worst-case scenario. This involves selecting a reference-sample material with a demonstrated high susceptibility to hydrogen embrittlement, and heat treating it to intensify this sensitivity.
The sample material is air-melted AISI 4340 steel heat treated to Rc 51 to 53 hardness. A test specimen for pinpointing hydrogen-embrittlement sensitivity is a 0.4 X 0.4-in. bar 2.2 in. long, with a notch in the middle of one side. The notch has a 90? angle, is 0.14 in. deep, and typically has a root radius of 0.020 in. for automotive testing and 0.010 in. for aerospace applications. Test samples, by design, have consistent fracture strength. Each certified lot is manufactured from the same batch of steel and heat treated at the same time.
The samples undergo a gradually increasing load, with a time delay between each step to permit hydrogen migration. This process continues until bending failure. The Rising Step Load test uses a four-point loading fixture and sensitive load cells and software to detect the onset of microcracking as well as the failure point.
To measure the effects of black oxide, technicians first tested an uncoated specimen to establish a reference base for applied load and number of steps to failure. The remaining bars from the certified lot were black-oxide coated, at the same time in a single production run.
Because heat-treated AISI 4340 specimens exaggerate the effects of hydrogen embrittlement, ASTM standards require that coated samples must maintain a fracture strength of at least 75% of the uncoated reference, to demonstrate the process is safe for fasteners. Actual results for the black-oxide coated samples were in the high 90% range, indicating that with respect to hydrogen embrittlement, the process passed with flying colors. (Complete test results are available from the author at [email protected].)
Users should note that the black-oxide process varies, to a degree, from one manufacturer to another. MIL-C13924 Class 1 describes the basic procedures that most processors use. However, actual temperatures of the various solutions, the type and concentration of acid, and the cleaners may differ.
The test results shown here, in general, give a clean bill of health for black oxide, but prudent practice is to test and certify products prior to use, particularly for critical applications.
CERTIFIED FRACTURE STRENGTH (lb)
ACTUAL FRACTURE STRENGTH (lb)
% FRACTURE STRENGTH
The table shows results of three certified specimens that were black-oxide coated and tested to failure. The specimens were tested using the Rising Step Load method, where stress increases 5% of the specimen's fracture strength every hour, and is held at that level until the next step, or failure. ASTM standards require that coated samples maintain a fracture strength of at least 75% of the uncoated reference.