Our customer is a global supplier of aerospace products. They approached us with a technical question around the sensitivity of oil-based versus water-based magnetic particle inspection inks.
Typically, their work involves the inspection of alloy steel gears and stainless steel pistons and shafts.
Our customer wanted to inspect for sub-surface faults in 17-4PH steel. This grade of steel is used in applications that demand high strength and good corrosion resistance, and lends itself to well to aerospace applications.
They wanted to understand whether differences in sensitivity performance would be seen between a water-based ink and an oil-based ink. They had been using MAGNAGLO® WB-12 and wanted to ascertain if the water-based carrier would allow more movement of the particles than the oil-based equivalent, MAGNAGLO® 14HF. One of their recently qualified inspectors had found a reference in their notes that water-based carriers form quicker indications, but didn’t know if this was actually true.
MAGNAGLO WB-12 and MAGNAGLO 14HF are based on the same magnetic particle – MAGNAGLO 14A – and are rated at the same SAE sensitivity: 7 – 8.
The primary reason for choosing water-based ink over versus oil-based ink is normally historical. Traditionally, oil-based inks were used because they have no associated corrosion issues. Coupled with this, the ready-to-use inks are simple to use, particularly in aerosol format where there is no need to carry out settlement volume checks.
In more recent times, companies have shifted to using water-based inks primarily for health, safety and environmental reasons: they produce no fumes, no fire risk and are easier to dispose of. The risk of corrosion during MPI testing has been eliminated through the incorporation of an inhibitor with the ink. In addition, water-based inks are easier to subsequently remove from the components being inspected.
Magnaflux EMEA provided the customer with some aerosol samples of 14HF so they could carry out side-by-side testing with both variants.
During the customer’s side-by-side tests, no significant differences were seen between the two different ink formats. There was some thought that the WB-12 may have been slightly more sensitive than the 14HF on some of the very fine subsurface discontinuities. However, these discontinuities were insignificant in terms of the overall product quality and did not influence the pass/fail criteria that they work to.