The 6351-T6 history
Aluminum alloy 6351-T6 was a popular cylinder material in the 1970s and 1980s. The alloy combined good strength with manufacturability, and major SCUBA and SCBA cylinder makers including Luxfer and Walter Kidde used it extensively. Roughly 1972 to 1988, depending on manufacturer, was the production window.
Over time, field experience surfaced a problem. A small but non-zero number of 6351-T6 cylinders developed cracks in the neck and shoulder, sometimes with catastrophic consequences. The failure mode is called sustained load cracking(SLC). Major manufacturers transitioned to a different alloy (6061-T6, which doesn't exhibit the same SLC behavior) by the late 1980s, but a large existing inventory of 6351-T6 cylinders remained in service and continues to be requalified today.
Sustained load cracking
SLC is a slow, time-dependent crack growth phenomenon in certain aluminum alloys under sustained tensile stress. The cracks initiate at the cylinder neck or shoulder, where the metal is thinnest and the stresses from filling are highest. Once a crack has started, it can extend over years of service, eventually reaching the threads and threatening cylinder integrity.
SLC is insidious because:
- The cracks start internal, in the neck threads, where they aren't visible during external visual inspection
- Cylinders with SLC may pass standard hydrostatic testing for years before the crack progresses to a critical size
- The growth is gradual; there's no specific trigger event to look for in service history
This is the failure mode that eddy current examination is designed to catch.
What §180.209(m) requires
Per 49 CFR §180.209(m), DOT-3AL cylinders made of aluminum alloy 6351-T6 used in:
- SCUBA (self-contained underwater breathing apparatus)
- SCBA (self-contained breathing apparatus)
- Oxygen service
must be requalified and inspected for sustained load cracking. The examination procedure specified in §180.209(m) is a combination of:
- External visual inspection per CGA Pamphlet C-6.1
- Eddy current examination in accordance with Appendix C of Part 180
The eddy current must be applied from inside the cylinder neck to detect SLC that may have extended into the neck threads. The condemnation criterion is strict: any crack in the neck or shoulder of two thread lengths or more, and the cylinder is condemned.
Cylinders used for industrial-gas service (CO₂, fire extinguisher, other industrial gases) are explicitly out of scope for the eddy current rule. The SLC risk in those services is much lower and §180.209(m) does not apply.
How eddy current works
Eddy current testing is an electromagnetic method for finding cracks and other discontinuities in conductive materials. A coil in the test probe carries alternating current, which generates a magnetic field that induces small circulating currents (the eddy currents) in the cylinder metal it's near.
When the probe passes over uniform metal, the eddy currents form a stable pattern. When the probe passes over a crack or other discontinuity, the eddy current pattern is disrupted. The probe detects this disruption as a change in its impedance (the relationship between voltage and current in the coil), and the eddy current instrument displays it as a signal on a screen or meter.
Cracks oriented across the eddy current flow produce the strongest signals. SLC cracks in cylinder necks typically run vertically, perpendicular to the circumferential eddy current flow when the probe is rotated in the neck. That orientation makes them detectable by a properly-applied eddy current scan.
The test procedure
An eddy current examination of a 6351-T6 cylinder follows these high-level steps:
- Visual inspection first. The cylinder is removed from service, depressurized, and externally inspected per CGA C-6.1. If it fails visual, no further testing is needed.
- Standardize (calibrate) the eddy current system.The technician runs the probe over a calibration standard with known reference notches. The instrument's response to the known reference establishes the baseline for evaluating actual findings on the cylinder.
- Insert the probe into the cylinder neck. The probe is positioned to scan from inside the neck, where SLC initiates.
- Rotate and scan. The probe is rotated through the cylinder neck circumference while readings are taken. Modern automated systems do this with motorized fixtures; older manual systems require the technician to rotate the probe by hand at controlled speed.
- Evaluate the trace. Any signal above the calibration threshold indicates a possible discontinuity. The technician evaluates the size, location, and orientation of any indications.
- Document the result. Pass or condemn, with the eddy current findings recorded alongside the visual and hydrostatic test data.
The technician performing eddy current must be familiar with the equipment and competent in standardizing it. The eddy current instrument's standardization is itself a calibration step that has to happen before every shift or job.
Reading findings
An eddy current trace on a healthy cylinder is mostly flat. Small variations from manufacturing texture and alloy composition are normal. A real crack indication shows as a distinct deflection from the baseline, often with a characteristic shape that experienced technicians recognize.
Per §180.209(m), the condemnation rule is concrete: any crack in the neck or shoulder of two thread lengths or more. Smaller indications may warrant further evaluation but don't automatically condemn the cylinder. Larger indications do.
Findings are recorded with:
- Indication location (which thread, which clock-position around the neck)
- Indication size (estimated crack length)
- Eddy current signal magnitude relative to the calibration reference
- The technician's judgment: condemn, evaluate further, or pass
These records become part of the cylinder's permanent history and are reviewed at audit if the cylinder's status comes into question.
VIP+ as a service
Eddy current isn't just for 6351-T6 cylinders. Many shops that have invested in the equipment offer eddy current as a value-added service on cylinders where it isn't strictly required, including non-6351-T6 aluminum and steel. The branding varies (often called "VIP+" in the dive industry, meaning Visual Inspection Plus eddy current).
The customer benefit is reassurance: the inspector has positively confirmed there's no hidden neck-thread crack waiting to grow. The shop benefit is a higher-margin service line using equipment already on the floor.
The bottleneck for a VIP+ program isn't the test. It's the documentation, retention, and audit-traceability of the eddy current records alongside the visual and hydrostatic records on each cylinder. Software handles the documentation overhead; the inspector handles the test.
