Abstract No. A67
CE
TA
S
Ana Isabelle Farinha
CRM Group
20
15
Determination of metallic coating weights by gravimetric methods:
uncertainties and limitations
20
15
24.06.15
Determination of metallic coating weights by
gravimetric methods: uncertainties and limitations
Determination of metallic coating weights
❚ 
❚ 
❚ 
❚ 
❚ 
CE
❚ 
Context
Specifications/requirements/standards
Methodology
Reproducibility of methodology (uncertainty)
Limitations
Scope of accreditation
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
TA
S
CETAS, 20th May 2015
2
1
❚ 
A coating of zinc or zinc alloy on iron or steel articles provides protection
against corrosion.
❚ 
The degree of protection is proportional to the weight [mass] of the
coating.
❚  Another aspect is the effect of coating weight on the
cost of the product. Zinc and other elements can
fluctuate in price and make a significant difference to
the price of galvanized sheet.
❚ 
For these and other reasons controlling the coating weight as well as other
product characteristics (thickness, strength, formability, the surface
treatment applied to the galvanized coating) are very important issues.
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
Context
20
15
24.06.15
3
TA
Context
CE
Designators for Zinc-based Coatings on steel sheet - SUMMARY
Some European or International standards describe the specifications or the
designation of different classes of coating :
CETAS 2015
European standard with requirements
for hot-dip coated products:
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
4
2
Some standard test methods for determining the weight [mass]
of coating for comparison with specification requirements
Few standards give indication about uncertainties, precision, bias.
No reference materials.
From ISO 17925:2004 - Zinc and/or aluminium
based coatings on steel - Determination of coating
mass per unit area and chemical composition
A planned trial of this method was carried out by 15
laboratories in 9 countries at 9 levels of mass per
unit area.
The precision obtained for hot-dip coated steel was
found to be insufficient. This was due, not to the
measurement method, but to low homogeneity of
samples, which were prepared from commercial
coating steel. Therefore the precision data obtained
for hot-dip coated sample are merely informative.
From ASTM A90 - Standard Test Method for
Weight [Mass] of Coating on Iron and Steel
Articles with Zinc or Zinc-Alloy Coatings
Bias - At the time of the study, there was no
accepted reference material suitable for
determining the bias for this test method,
therefore no statement on bias is being made.
5
TA
Determination procedure
Method for determining the coating mass on Zn alloy coatings on steel.
We also present the procedure to estimate the uncertainty of the
measurements.
This are the coatings on which we apply this method :
CE
Zn
Zn - Al
Zn - Fe
Zn - Fe - Al
Zn - Mg – Al
Zn - Al - Si
Al - Si
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
Context
20
15
24.06.15
6
3
Determination procedure
Sample preparation
❚ 
Test specimens will be of round shape with a surface area of 45
mm of diameter. At least two test specimens shall be taken
from a representative sample piece (coated sheet).
Procedure
❚ 
❚ 
❚ 
Clean the specimens with suitable solvent and dry it.
Weigh the sample with a balance capable of weighing samples
to an accuracy of 0.01mg (m0).
Place the sample in a sealed dissolution cell.
7
TA
Determination procedure (cont.)
Add the attack solution (HCl + HMTA inhibitor) at ambient temperature.
Leave the sample immersed in the solution until complete dissolution
(no bubbling).
❚ 
After the attack, wash the sample under distilled water and dry it with
a cloth.
❚ 
Weigh the sample again to an accuracy of 0.01mg (m1)
❚ 
Determine the difference between the mass of the coated sample (m0)
and that of the sample without its coating (m1).
CE
❚ 
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
8
4
Determination procedure (cont.)
❚ 
Measure the diameter (D) of the sample with a calliper to the nearest
0.05mm . At least two measurement shall be taken.
❚ 
The coating mass in grams per square metre of the product is obtained
from the formula:
𝑐𝑜𝑎𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 (​𝑔/​𝑚↑2 )= ​(​𝑚↓0 −​ 𝑚↓1 )/𝜋 × ​(​𝐷/
2 )↑2 m0 and m1 are the mass, in grams, of the sample before and after stripping;
D is the diameter, in millimetres, of the exposed surface of the sample.
❚ 
❚ 
❚ 
This coating mass is obtained per face, separately, the sealed dissolution
cell is capable of protecting one side of the coated piece of steel sheet
while the other side is being stripped in HCl solution.
9
TA
Uncertainty estimation
The process of measurement uncertainty estimation *:
Step 1. Specify measurand
Step 2. Identify uncertainty sources
Step 3. Quantify uncertainty components
CE
Step 4. Calculate combined uncertainty
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
* EURACHEM/CITAC Guide CG 4
“Quantifying Uncertainty in Analytical Measurement”
CETAS 2015
10
5
Uncertainty estimation
❚ 
Calculate standard uncertainty for each parameter that can have an
influence on the final result.
Two uncertainty types:
❚  Type A : estimate of input quantity obtained by repeat measurements
 Type B: use data from any other source available
❚  certificates (CRMs, balance calibration, …)
❚  manufacturer’s specifications (balances, volumetric flasks, …)
❚  previous measurements (validation data, …)
❚  uncertainties of reference data
11
TA
Uncertainty in our measurements
Step 1. Specify measurand
𝑐𝑜𝑎𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 (​𝑔/​𝑚↑2 )= ​(​𝑚↓0 −​ 𝑚↓1 )/𝜋 × ​(​𝐷/2 )↑2 m0 and m1 are the mass, in grams, of the sample before and after stripping;
D is the diameter, in millimetres, of the exposed surface of the sample.
CE
Step 2. Identify uncertainty sources
1.  In coating dissolution
Not possible to quantify without a perfect homogeneity of samples
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
12
6
Uncertainty in our measurements
Step 2. Identify uncertainty sources ( cont.)
2. In mass
Uncertainty for mass (m0 and m1)
- uncertainty from balance calibration certificate (ucal)
- from previous measurements (uCRM)
um= √(ucal2 + uCRM2)
The component of mass in the combined uncertainty wil be added two times (for
the sample is weighed twice) 13
TA
Uncertainty in our measurements
Step 2. Identify uncertainty sources ( cont.)
3. In diameter
Uncertainty for diameter (D)
- uncertainty from caliper calibration certificate (ucal)
CE
- from previous measurements (uCRM)
- uncertainty from the roundness (uround)
(uD)= √(ucal2 + uCRM2+ uround2)
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
14
7
Quantifying uncertainty in our measurements
Step 3. Quantify uncertainty components
Standard uncertainty of mass : usm= √(um02 + um12)
Standard uncertainty of diameter : (uD)= √(ucal2 + uCRM2+ uround2)
Step 4. Calculate combined uncertainty (model involving
product or quotient of quantities)
​𝑢↓𝑚𝑎𝑠𝑠 /𝑚𝑎𝑠𝑠 =√⁠(​ ​𝑢↓𝑠𝑚 /𝑚 )↑2 +(​𝑢↓𝐷 /𝐷 )↑2 + (​𝑢↓𝐷 /𝐷 )↑2 15
TA
Expanded uncertainty and coating mass
For normally distributed values and sufficient degrees of freedom: k=2 (95%
conf.), k=3 (99% conf.)
Coverage factor k: level of confidence required
To calculate the expanded uncertainty (U)
Umass = 2 * usm = +/- X g/m2
CE
So the coating mass will be expressed as :
Coating mass = X +/- Umass g/m2
(per unit area of sheet on single side weight)
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
Expanded uncertainty with a coverage factor k=2 corresponding to a level of confidence of about 95 %
estimated in accordance with the Guide to the Expression of Uncertainty in Measurement (GUM)
CETAS 2015
16
8
Application to one coating mass
❚ 
❚ 
M0 = 12.65g
M1 = 12.57g
D = 41.21 mm = 0.04121 m
𝑐𝑜𝑎𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 (​𝑔/​𝑚↑2 )= ​(​𝑚↓0 −​ 𝑚↓1 )/𝜋 × ​(​𝐷/2 )↑2 = 56 .75 g/m2
❚ 
❚ 
❚ 
Standard uncertainty of mass (usm)= √(uM02 + uM12) = 0.250mg
Standard uncertainty of diameter (uD)= √(ucal2 + uCRM2+ uRound2 = 0.40mm
​𝑢↓𝑚𝑎𝑠𝑠 /𝑚𝑎𝑠𝑠 =√⁠(​ ​𝑢↓𝑠𝑚 /𝑚 )↑2 +(​𝑢↓𝐷 /𝐷 )↑2 + )↑2 = 0.0141= 2 * uu =0.014
* 56.75 = 0.80 g/m
(​𝑢↓𝐷 /𝐷 Expanded uncertainty : U
1.60 g/m
mass
mass =
mass
2
2
Coating mass = 56.75456 +/- 1.60 g/m2
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
(per unit area of sheet on single side weight)
17
TA
Limitations
Some limitations and requirements are to be taken in
consideration :
- Homogeneity and cleanness of the sample (the coating surface
shall be visually inspected and the sampling shall be defect-free).
- No surface treatment (phosphatation ; conversion … )
CE
❚ 
- No heat treatment
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
18
9
Heat treatment limitation
After stripping (sample annealed at 900°C) difference in faces:
- face A - no Zn residual
- face B - important residual of Zn
Before stripping 1,2 1 0,8 After stripping
Zn1_FB mV Fe2_FB 0,6 Zn1_FA Fe2_FA 0,4 0,2 1E-­‐08 0 500 1000 $me (sec) 1500 ©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
2000 19
TA
Heat treatment limitation
in XRD, faces are different
dont show peak of usual Fe-Zn phases
CE
-  face A peak of ferrite
-  face B peak of ferrite with secondary peaks that indicate a solid solution
A
B
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
20
10
The phase diagram (Fe-Zn) shows that this solid solution is thermodynamically
possible if the annealing is higher than 782°C.
At > 782°C,
a solid-liquid reaction
takes place and a ferrite layer
with Zn in solution is present
Problem from the stripping
If the inhibitor works in this
diffusion layer
the stripping will stop and
the Zn present in this layer
will not be counted
21
TA
Accreditation for the coating weight (mass)
The laboratory of the CRM Group is accredited in accordance with the
recognised International Standard ISO/IEC 17025 for determining the
coating weight by gravimetry.
❚ 
A laboratory's fulfilment of the requirements of ISO/IEC 17025 means
the laboratory meets both the technical competence requirements
and management system requirements that are necessary for it to
consistently deliver technically valid test results.
CE
❚ 
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
22
11
Scope of accreditation for coating
weight (mass)
Determination of coating mass, per face, in hot-dip and electrogalvanize steel
coatings :
Zn ; Zn - Al ; Zn - Fe ; Zn - Fe - Al ; Zn - Mg - Al ; Zn - Al - Si ; Al - Si
Coa$ng weight per face +/-­‐ (**) Standard uncertainty (u) : u Scope Min (g/m2) Max (g/m2) 30 0.91 300 8.33 0.46 = 4.17 (**) Expanded uncertainty expressed with a coverage factor k=2 ((+/-­‐) = k x u) Some limitations, linked to samples (homogeneity, cleanness, no surface treatment,
- phosphatation ; conversion …,no heat treatment) are given by request.
23
CE
TA
S
CETAS 2015
©CRM Group - All rights reserved for all countries. Cannot be disclosed, used, or reproduced without prior written specific
authorization of CRM Group - CONFIDENTIAL - Privileged Information - CRM Group proprietary information.
20
15
24.06.15
12