May 11, 2009

Surface Preparation: It’s A “Blast” – Part I

by Tony Ippoliti

The cover of the August 2007 issue of Modern Steel Construction highlighted an article within its pages: “Making Steel“. A Google search of the words “making steel” delivers over 61 million hits. One of these, Key-to-Steel,  provides several examples of the steel-making process.  Fascinating.  There’s much information available about the making of steel.

August 2007 Cover

Briefly, “virgin” steel is made by introducing energy – in the form of heat – to vessels that contain iron ore and other ingredients. These ingredients melt and are subsequently formed into shapes of various widths, lengths, and thickness. These shapes are then fabricated into the tanks, pipe, and process equipment with which we’re all familiar.

Unfortunately, the energy used in steel-making process leaves an oxide on the surface of steel – called mill scale – that must be removed prior to fabrication into steel tanks, filter equipment, and pipe, for example. Mill scale is a “cathodic” layer that will cause the steel substrate beneath it to corrode.  The simplest way to remove mill scale, prior to fabrication, is by abrasive blasting in an enclosure or by a trained worker.  The Society for Protective Coatings and NACE International have produced several joint standards for this purpose.

For submerged or buried surfaces, most coating manufacturers recommend abrasive blasting in accordance with the SSPC-SP 10/NACE 2 Standard “Near White Blast Cleaning“; for atmospheric use, SSPC-SP 6/NACE 3 “Commercial Blast Cleaning“.  Both standards demand the complete removal of mill scale and other contaminants.  The difference between these two standards is the level of surface cleanliness, SP 10 having the higher level.  SSPC and NACE also have a joint standard (Vis 1) that visually depicts how an SP 10 and and SP 6 should appear . Vis 1, “Visual Standard for Abrasive Blast Cleaned Steel“,  helps to inspect / verifiy abrasive blasted steel surfaces.

Upcoming in Part II: power tools and creating the proper surface profile.

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