Using
Magnetic Coating Thickness Gauges
to Measure the Thickness of
Galvanizing

 

 

By David Beamish

DeFelsko Corporation, Ogdensburg, NY

Based on a PowerPoint  presentation made to:
American Galvanizing Association
   (October 2003)
   Kansas City
This article provides an overview on how Coating Thickness Gauges are used within the galvanizing industry. It describes gauge choices, the 4 measuring steps, several precautions to be taken, and a Q&A sections for commonly asked questions.

Helpful information:

 

Other related products:

BACKGROUND

Steel fabricated parts are galvanized to provide corrosion protection. The final step in the process is inspection of...

  • Coating Thickness

  • Visual Appearance

  • Adherence

  • Uniformity

 

The thickness of the galvanized coating is directly related to…

  • Service life

  • Amount of corrosion protection

  • Quality

 

The thicker the coating, the longer the service life. Thus, coating thickness is the single most important inspection check to determine the quality of a galvanized coating.

 

 

 

MEASURING THICKNESS

Size, shape and number of pieces to be tested dictates the method to be used. Specified test methods are either destructive or non-destructive. There are four ways to measure the thickness of galvanizing...

  1. Magnetic Thickness Gauges

  2. Stripping & Weighing

  3. Weighing Before & After Galvanizing

  4. Optical Microscopy (ASTM B 487)

 

The most practical test is the non-destructive method utilizing the magnetic principle for determining coating thickness. It is...

  • Non-destructive

  • Simple, Fast and Inexpensive

  • Complies with recognized international standards including ASTM E 376, CSA G 164-M, ISO 2808

MAGNETIC THICKNESS GAUGES

The 3 most common types of thickness gauges fall into one of two categories...

 

Because it is non-destructive, magnetic thickness measurements are the most common method of evaluating galvanizing coating thickness.

 

COMPARISON

 

Mechanical Gauges

  • measure the strength required to pull a magnet away from steel. The thicker the zinc, the weaker the attractive magnetic force.

Electronic Gauges

  • measure the change in flux density using electronic circuitry.
  • digital readout
  • various probes
  • memory
  • Permanently calibrated
  • Simple and Rugged
  • + 5% accuracy
  • $350
  • Permanently calibrated with the ability to be specially calibrated
  • + 1% accuracy
  • $600+

Coating thickness is the single most important inspection check to determine the quality of a galvanized coating.

MECHANICAL GAUGES

Pen-Style:

  • Permanently calibrated

  • Smallest unique magnet

  • Perfect for small, hot or hard-to-reach areas

  • + 10% accuracy

PosiPen

 

Dial-Type:

  • Permanently calibrated

  • Simple, Rugged, Universally accepted

  • Explosion Proof

  • + 5% accuracy

PosiTest

ELECTRONIC GAUGES

  • Fast & Simple

  • Auto or Manual Calibration

  • Easy-to-read digital display

  • Versatile – various built-in or separate probes

  • Options -printing, charting

  • Statistical capabilities – averaging, min/max

  • + 1% accuracy

PosiTector 6000

     

MEASURING STEPS

  1. Calibrate – an operation that is typically performed by the manufacturer or by a qualified lab.
  2. Verify - an accuracy check performed by the user using known reference standards.
  3. Adjust - align the gauge’s thickness readings to match those of a known sample. For a dry film thickness gauge this would mean adjustment to zero on the uncoated surface or to known thicknesses such as shims.
  4. Measure - measure the zinc coating at the number of spots as specified in ASTM A123.
 

 

1. CALIBRATE

  • the act of measuring known Calibration Standards and verifying that results are within the tolerance of the Gauge.
  • an operation that is typically performed by the manufacturer or by a qualified lab.
  • user specific calibration intervals should be established based upon frequency of use, handling practices and measuring environment.

 

2. VERIFY

  • an accuracy check performed by the user using known reference standards. Intervals should be established based upon frequency of use, handling practices and measuring environment.

 

3. ADJUST

  • an adjustment, or Calibration Adjustment, is the act of aligning the Gauge’s thickness readings to match those of a known sample in order to improve the effectiveness of the Gauge on a specific surface or in a specific portion of its measuring range.
  • One point or two point calibration adjustments are possible.
  • For an electronic gauge this would typically mean adjustment to zero on the uncoated surface or to known thicknesses such as shims.

For electronic gauges, choose 1 of 3 methods…

  • Adjust to zero on the uncoated part.
  • Adjust to a known thickness such as a shim.
  • Adjust at 2 points, one point below and one point above the expected thickness range.

 

 

 

 

 

 

Why adjust?

 

To compensate for… 
  • Magnetic properties of the steel
  • Thickness of the steel
  • Proximity to an edge
  • Curvature
  • Roughness of the steel surface
  • Residual magnetism in the steel
  • Other magnetic fields

 

 

 

 

 

 

 

The PosiTector 6000 is factory calibrated and performs an automatic self-check each time it takes a measurement. For many applications no further adjustment is necessary after a RESET. Just check ZERO on the uncoated steel substrate, then measure.

CAUTION:

When shims are used, resultant gauge measurements are less accurate and must be recalculated. For example, say the accuracy of a properly calibrated gauge is ±2%. The thickness of a shim might be accurate to within ±3%. The combined tolerance of the gauge and the shim will be ±4% as given by the sum of squares formula:

 

4. MEASURE

Using a mechanical pull-off gauge… 
  1. Measure the bare substrate at a number of spots to obtain a representative average value. This average value is the base metal reading (BMR).
  2. Measure the zinc coating at the number of spots specified in ASTM A123
  3. Subtract the base metal reading from the gauge reading to obtain the thickness of the zinc.

Using an electronic gauge… 
  1. Adjust to zero on the uncoated substrate.
  2. Check by measuring shims placed on the uncoated substrate.
  3. Measure.

PRECAUTIONS

  • follow manufacturer’s instructions
  • verify calibration regularly using reference standards
  • the test surface should be free from dirt, grease, oxide and corrosion products
  • test points should be chosen to avoid obvious peaks or irregularities in the coating
  • a sufficient number of readings should be taken to obtain a true average

CONVERSIONS

ASTM  STANDARDS

Excerpts from ASTM A123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products...

  • The specimen coating thickness shall be the average of a minimum of 5 readings at widely dispersed points.
  • This average value shall be not less than one coating thickness grade lower than the value listed in the appropriate specification.
  • Thickness should be between 1.4 and 3.9 mils depending on the coating grade.

ASTM E 376 Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Examination Methods

ASTM A123/A123M-12 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products

ASTM A153/A153M-09 Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware

ASTM A653/A653M-09a Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process

A767/A767M-09 Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement

DUPLEX COATINGS

There are three ways to measure the thickness of paint over galvanized steel …

  1. Adjust to zero on the zinc surface before painting.

Use an electronic magnetic gauge such as the PosiTector 6000 F1 and “zero” it on the surface of the zinc. Then measure the paint thickness only. For best results it is recommended that several readings be taken to obtain an average paint thickness.

PLUS:  Simple. Gauge displays paint thickness only.

MINUS: Only some electronic gauges have this capability and the Gauge doesn’t know which layer is responsible for changes in thickness.

  1. Use an eddy-current gauge.

These gauges measure the thickness of non-conductive coatings over conductive substrates. Zero the gauge on the zinc, then measure the paint thickness directly.

PLUS:  Zinc thickness variations usually don’t affect paint thickness results if the zinc is >3 mils

MINUS:  -Zinc thickness variations do affect paint thickness results if zinc is thin.

                -Gauge is overly sensitive to roughness, magnetism, curvature, etc.

                -Practical guidelines do not exist to guide the use of eddy current gauges on ferrous substrates

  1. BEST METHOD: Measure before and after the item is painted.

Use a magnetic gauge such as the PosiTector 6000 F1 or PosiTest FM to measure the thickness of both the zinc and paint layers together. Subtract the zinc layer thickness to provide the paint thickness value.

PLUS:  Fast and simple for any magnetic gauge.

MINUS:  Gauge doesn’t know which layer is responsible for changes in thickness.

 

 

 

 

 

 

Q & A

Why does it seem I never get the same reading twice?

Although the surface may “appear” smooth, microscopic surface roughness exists on both the zinc and steel. Therefore, the best representation of coating thickness is obtained by averaging a series of measurements as per ASTM A123.

What should a coating thickness gauge read on a G90 galvanized coating?

According to ASTM A 653-03, a G90 coating designation means the weight of zinc on both sides of a steel sheet is 0.90 oz/ft2

A magnetic thickness gauge measures only one side.

Therefore:

  • 0.45 oz/ft2 x 1.684 = 0.76 mils per side
  • 0.45 oz/ft2 x 42.78 = 19 microns per side

G115 = 0.97 mils (25 um) / side

G60 = 0.51 mils (13 um) / side

G40 = 0.34 mils (9 um) / side

G30 = 0.25 mils (6 um) / side

Other Related Products

 

ULTRASONIC WALL THICKNESS GAUGES

Simple, hand-held devices like the PosiTector UTG are ideal for measuring the total thickness of various metal objects including kettle wall thicknesses.

They measure the wall thickness of materials such as : steel, cast iron, plastic and more. Ideal for measuring the effects of corrosion or erosion on tanks, pipes or any structure where access is limited to one side.

ADHESION TESTERS

Our PosiTest adhesion tester is not the preferred method for measuring the bond strength of galvanizing. The following are excerpts from ASTM A123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products...

  • Adhesion of the zinc coating to the surface of the base metal is usually performed by cutting or prying with the point of a stout knife, applied with considerable pressure in a manner tending to remove a portion of the coating.
  • The adhesion is considered inadequate if the coating flakes off in the form of a layer of the coating so as to expose the base metal in advance of the knife point.