The first line of attack would be ultrasonic wall thickness measurement. To carry out wall thickness measurements with ultrasonics the pipe would have to be comfortably accessible and 'bare metal' i.e. free of coating, insulation, rust etc. Typically, measurements can be made - assuming we are talking of carbon steel - with an accuracy of 0,2 mm or so in the field. Oviously an asessment of the situation on site would be required.
If the line is insulated, an alternative method of measurement, without removing the insulation, might be Incotest. Incotest is a stand-off measurement system which can 'look' through insulation and assess the remaining wall thickness compared to one point where the insulation has been removed and a comparision is made.
You mention pitting. I wonder, considering the medium in the line (iron ore slurry), if there really is a problem with pitting. I tend to think that the problem would be erosion. However, if pitting is present and this is local loss of wall thickness rather than needle pitting, this should be possible to detect.
Needle pitting, which is usually a problem with exotic steel sorts, is very difficult to detect with either of the above methods (read impossible). Radiography may work.
Another method that you may wish to consider is SLOFEC, this is an eddy current based technique which uses magnetic saturation to aid the induction of eddy currents into the specimen. This is an excellen tmethod for fast screening for corrosion typical of what you have described.
You may want to consider applying an Ultrasonic (UT) based Intellingent Pig technology. Even if the line is not designed to be inspected by way of intelligent pigs, there are pigs which are capable of running in these lines. Some Ultrasonic based intelligent pigs will provide +100% overlapping coverage to ensure the entire surface is inspected. Detecting and quantifying Corrosion, Pitting, Erosion along with any deformation (i.e. Denting, Ovality, Bulging, etc.) can be accomplished with one pass of the intelligent pig tool.