Magnetic particles, their characteristics and application

Abstract

While a great deal of attention is paid to ensuring that magnetic fields of suitable intensity and direction are induced into components and materials which are tested by magnetic particle inspection, the actual particles used are largely taken on trust. Nowadays, the choice of particles is much greater than was once the case. Until relatively recently the choice of particles was between materials, the ‘wet’ and ‘dry’ methods, either of which might be of a colour-contrast type or fluorescent and, in the case of the wet method, whether to use an aqueous or oilbase carrier fluid. The fluorescent materials then offered appeared bright yellow/green when viewed under UVA (black light) of 365 nm wavelength. Differences in fluorescent brilliance were fairly minor while other characteristics were less uniform from one type to another. At present we have a choice of colour in the fluorescent range which includes true yellow, the classical yellow/green, true greens and various shades of orange to red. Also within this range, levels of fluorescent brilliance vary; brilliance of up to five times that of the standard fluorescent materials is now available. The magnetic materials which are used for the particles vary too. The standard fluorescent magnetic materials were based on one of the magnetic iron oxides whereas many modern materials use extremely fine iron particles, either completely or in combination with iron oxide.