Well integrity was and continues to be a significant priority for many operating companies around the world. A compromise in well integrity may have direct impacts on production sustainability well life and the environment. Well integrity became an important topic in the last decade after a number of the well blow-outs and oil spills around the world. The new electromagnetic time domain tool (EM) which is presented uses transient or Pulsed Eddy Current (PEC) measurements to perform quantitative evaluation of downhole corrosion in four concentric tubulars individually and to inspect a fifth tubular qualitatively. Case studies are presented that compare results of this instrument with industry-standard single-string evaluation tools such as multi-finger calipers. The novel electromagnetic tool which uses transient or PEC technology comprises three sensors which achieve high-resolution of the inner barrier and high radial depth of investigation for up to five barriers. Each sensor induces coaxial rings of eddy currents in multiple concentric tubulars and measures a time-varying response from the outward-diffusing eddy currents. The full transient responses from multiple sensors are then interpreted to obtain individual tubular thickness profiles. Case studies are presented where electrochemical external corrosion has penetrated inward and has affected the inner most barrier by having through holes which are also verified with another high resolution Multifinger caliper tool. Individual thickness measurement is valuable for proactive well integrity management because electrochemical external corrosion which is the primary corrosion mechanism in these wells causes the outermost casing to fail first and then continues to penetrate inwards. Therefore the new electromagnetic instrument enables early diagnosis of the outer tubulars to identify potential weak zones in the completion string while logging through tubing and eliminating the cost of pulling completions for this purpose. The paper covers the basics of corrosion logging tools and the benefits and drawbacks of running various tools and the advantages of combining several together. Also how the tubing completion like packer setting depths can affect the production casing integrity as well as workover operation impact. New case studies with multi-finger calipers support these conclusions.