Detecting & Identifying Defects in Steel During Surface Preparation

Coatings shrink as they cure, resulting in low film thickness over sharp edges and welds, which is a classic cause of coatings failure.

Repairing defects is normally not part of an abrasive blaster’s job description, but when you are charged with prepping the surface it’s imperative the coating adheres properly, and surface defects can prevent that from happening. The abrasive blaster is the only person on the job who looks at every single square inch of the steel surface, which makes you the last line of defence for finding defects.

Finding and reporting defects is not only important, but welcome. Providing this additional service is a good way to make an impression and distinguish yourself from the competition. But first, you need to know what to look for.

Three Kinds of Steel Surface Defects

ISO 8501-3, “Preparation of Steel Substrates Before Application of Paints and Related Products”, sorts defects into three categories:

• Welds

• Edges

• Steel surfaces in general

When inspecting for defects, pay close attention to welded joints, cuts, punctures and scoring. Run a gloved hand over the distressed areas looking for places where it catches to identify protrusions. Some protrusions, such as weld spatter, can be abrasive blasted. Others will require grinding.

If you find yourself in the position where you are asked to grind a protrusion with power tools, it’s important to avoid leaving any surface roughing, burrs or burnishing. It’s also important to note when grinding defects to not reduce the mass of the substrate to less than that the surrounding metal, or grind in a way that creates excessive heat – both cases will weaken the steel. Also, grinded defects should be inspected afterwards by the glove test to ensure that more sharp edges were not created in the process.

Knowing what kinds of defects to look for, lets look at the different grades of surface prep.

Making The Grade

There are three grades for steel surfaces dealing with visible imperfections as set out by ISO 8501-3:

P1 Light Preparation: no preparation or only minimum preparation needs to be carried out before application of paint;

P2 Thorough Preparation: most imperfections are remedied;

P3 Very Thorough Preparation: surface is free of significant visible imperfections.

When deciding on the preparation grade, make sure you’re preparing the surface to specifications, especially considering that a single fabrication may require different prep grades — for instance: P1 on the inward-face and P3 for the outside wall.

Getting a consensus on between all relevant parties on preparation grades and specific visible imperfections before you start can save time and effort.

Imperfections, Defects and Prep Grades Revealed

Weld Spatter

During the transfer of wire to weld there is a disturbance in the molten weld pool. This disturbance, wusually caused by the voltage being too low or amperage being too high, causes weld-matter to fulminate onto the steel surface.

Preparation Grades

P1 - Fee of all loose weld spatter (a)
P2 - Free of all loose and lightly adhering weld spatter (a+b)
P3 - Surface must be free of all weld spatter. (a + b + c)

Weld Ripple/Profile

Surface oscillations in the weld pool are solidified as a ripple along the length of the bead profile.

Preparation Grades

P1 - No Preparation
P2 - Surface shall be dressed to remove irregular and sharp-edged profiles
P3 - Surface shall be fully dressed, i.e. smooth

Weld Slag

The deoxidization process between the flux coating, air, and surface; the result of which is deposited as a residue on and around the weld bead.

Preparation Grades

P1 - Surface shall be free from welding slag
P2 - Surface shall be free from welding slag
P3 - Surface shall be free from welding slag

Udercut

A weld flaw; a groove or crater near the toe of the weld-bead resulting in a weak bond, and is prone to cracking.

Preparation Grades

P1 - No Preparation
P2 - Surfacce shall be free from sharp or deep undercuts
P3 - Surface shall be free from sharp or deep undercuts

Weld Porosity

Weld Porosity is a common welding defect. When the torch is applied to treated metal, gasses are released and absorbed into the molten metal. As the metal cools, the gasses ar released from the metal, forming pores.

Preparation Grades

1 - Visible

2 - Invisible (might open after abrasive blast cleaning)

P1 - No Preparation

P2 - Surface pores shall be sufficiently open to allow penetration of paint

P3 - Surface shall be free from visible pores

Weld End Crater

Incorrect weld technique; this deffect occurs where the arc has been broken, resulting in a crater.

Preparation Grades

P1 - No Preparation

P2 - End craters shall be free from sharp edges

P3 - Surface shall be free from visible end craters

Rolled Edges

When an edge has been formed to transition from one surface plane to another surface plane in a gradual curvilinear fashion.

Preparation Grades

P1 - No Preparation

P2 - No Preparation

P3 - Edges shall be rounded with a radius of not less than 2 mm

Edges: Punch, Shear, Saw, Drill

Edge transitions made during fabrication by punching, shearing, sawing or drilling tools.

Preparation Grades

P1 - No part of the edge shall be sharp; the edge shall be free from fins

P2 - No part of the edge shall be sharp; the edge shall be free from fins

P3 - Edges shall be rounded with a radius of not less than 2 mm

Thermally Cut Edges

When a plasma, oxygen fuel, or other thermal process is used to cut steel.

Preparation Grades

P1 - Surface shall be free of slag and loose scale

P2 - No part of the edge shall have an irregular profile

P3 - Cut face shall be removed and edges shall be round

Pits And Craters

When corrosion, of an extremely localized variety, leads to small perforations on the steel substrate.

Preparation Grades

P1 - Pits and craters shall be sufficiently open to allow penetration of paint

P2 - Pits and craters shall be sufficiently open to allow penetration of paint

P3 - Surface shall be free of pits and craters

Shelling/Slivers/Hackles

A layer of corrosion in steel causting the corroded surface to separate and lift, leaving an interlocking flaky shell texture.

Preparation Grades

P1 - Surface shall be free from lifted material

P2 - Surface shall be free from visible shelling

P3 - Surface shall be free from visible shelling

Roll Overs/Cut Laminations

A fabrication defect cutting into the surface causing a thin protruding slice.

Preparation Grades

P1 - Surface shall be free from lifted material
P2 - Surface shall be free from visible roll-overs/cut laminations
P3 - Surface shall be free from visible roll-overs/cut laminations

Rolled-In Extraneous Matter

A defect ocurring during the fabrication process where foriegn matter is caught under a mechanical roller, and embedded into the steel surface.

Preparation Grades

P1 - Surface shall be free from rolled-in extraneous matter

P2 - Surface shall be free from rolled-in extraneous matter

P3 - Surface shall be free from rolled-in extraneous matter

Grooves And Gouges

A disfigured burrow or rough opening found in a steel surface, typically caused by mishandling.

Preparation Grades

P1 - No preparation

P2 - The radius of grooves and gouges shall be no more than 2 mm

P3 - Surface shall be free from grooves and gouges

Indentations And Roll Marks

A deep furrow or recess in the steel substrate, typically caused by mechanical manipulation.

Preparation Grades

P1 - No preparation

P2 - Indentations and roll marks shall be smooth

P3 - Surface shall be free from indentations and roll marks

Key Takeaway

Surface defects should be identified during the inspection, with a plan to remove them discussed before the job begins. However, if you encounter surface defects that look problematic during the course of blasting, report the defects to the project manager before taking action.