Chromium on HDG Steel

    Application Note EDXRF1904

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    The measurement of chromium (Cr) conversion coating on hot-dip galvanized steel is demonstrated.


    Aluminum and steel are often coated with a protective conversion coating, also called passivate or passivation coating, to prevent oxidation and corrosion of the base metal. Conversion coatings include chromium (Cr), titanium (Ti), vanadium (V), manganese (Mn), nickel (Ni), phosphorus (P), or zirconium (Zr). A phosphate coating may also be applied to minimize wear on cutting tools and stamping machines.

    Aluminum is often coated for use in aircraft parts, aluminum window frames, and other similar industries where the aluminum is exposed to weathering. Steel for the automotive industry is typically first galvanized with a zinc coating before the conversion coating is applied. Protected steel is also used for outdoor sheds and other similar uses where steel is exposed to weathering. Conversion coating also helps in the retention of paint for the final finished product.

    NEX QC+ high-resolution EDXRF analyzerModel: NEX QC+


    An empirical calibration was built using a set of standards assayed by careful weigh-strip-weigh. The thickness of the galvanized layer may not be constant, so an automatic correction can be enabled that compensates for the Zn X-rays effect on the Cr X-ray signal if the Zn coating varies.

    Element: Cr
    Units: mg/ft²
    Sample I.D. Standard value Calculated value
    STD 1 1.3 1.32
    STD 2 2.3 2.30
    STD 3 3.3 3.27
    STD 4 5.7 5.72

    EDXRF1904 Correlation plot Cr on HDG steelCorrelation plot Cr on HDG steel

    Recovery and repeatability

    To demonstrate repeatability (precision), the low and high calibration standards were chosen. Each sample was measured in a static position for ten repeat analyses with typical results shown below.

    Element: Ti
    Units: mg/ft²
    Sample I.D. Standard value Average value Std. dev % Relative
    STD 1 1.3 1.30 0.013 1.0
    STD 4 5.7 5.69 0.022 0.4


    The performance shown here demonstrates NEX QC+ provides excellent sensitivity and performance for the measurement of chromium conversion coatings on HDG steel.

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