de-icing materials and corrosion – laboratory testing report · corrosion of metals and alloys...

Highways Consultancy Group - Highways Research Group

De-icing Materials and Corrosion – Laboratory Testing Report

Task Reference: 628(387)MTSC Project Sponsor: Christopher Plumb Report Date: March 2009

Task Reference: 628(387)MTSC Laboratory Testing Report

This report is prepared by the Highways Consultancy Group/Highways Research Group (HCG/HRG). HCG/HRG is a Consortium of four core suppliers, namely Mott MacDonald, Scott Wilson, Owen Williams and CIRIA, supported by an extensive supply chain. The companies from the supply chain contributing to this report are:

• Scott Wilson Ltd The nominated Task Manager is: Robert Walker Tel: 01256 310544 Fax: 01256 310201 Email: [email protected] Address: Scott House, Alençon Link, Basingstoke, Hampshire RG21 7PP Issue and Revision Record Rev Date Originator Checker Approver Description --- 03/09 R Walker J Mills R Walker Draft A 03/09 R Walker J Mills R Walker Reissued with additional data B 04/09 R Walker J Mills R Walker Final This document has been prepared for the titled project or named part thereof and should not be relied upon or used for any other project without an independent check being carried out as to its suitability, and prior written authority of Mott MacDonald being obtained for all tasks carried out through the Technical Consultancy Framework; and prior written authority of Scott Wilson for all tasks carried out through the Research and Development Framework. Mott MacDonald or Scott Wilson accept no responsibility or liability for the consequence of this document being used for a purpose other than the purposes for which it was commissioned. Any person using or relying on the document for such other purpose agrees, and will by such use or reliance be taken to confirm his agreement to indemnify Mott MacDonald or Scott Wilson respectively for all loss or damage resulting therefrom. Mott MacDonald or Scott Wilson accept no responsibility or liability for this document to any party other than the person by whom it was commissioned. To the extent that this report is based on information supplied by other parties, Mott MacDonald or Scott Wilson accept no liability for any loss or damage suffered by the client, whether contractual or tortious, stemming from any conclusions based on data supplied by parties other than Mott MacDonald or Scott Wilson respectively, and used in preparing this report.

Task Reference: 628(387)MTSC Laboratory Testing Report

List of Contents

1. Introduction.................................................................................................. 1

2. Methodology................................................................................................ 2

2.1 Test procedure .........................................................................................................2

2.2 Limitations ................................................................................................................3

3. Results ........................................................................................................ 4

3.1 Aluminium.................................................................................................................4

3.2 Mild steel ..................................................................................................................5

3.3 Galvanised steel.......................................................................................................6

4. Discussion ................................................................................................... 9

5. Conclusions............................................................................................... 10

Appendix A – Experimental test results ................................................................A-1

Appendix B – Photograph of test samples............................................................B-1

Task Reference: 628 Page 1 Laboratory Test Report

1. Introduction

The Highways Agency (HA) has an obligation, where reasonably practicable, to prevent safe travel on the motorway and trunk road network in England being endangered by the presence of snow and ice. Historically this has been achieved by precautionary anti-icing using dry salt, also known as rock salt. More recently alternatives to dry salt have been adopted or are being trialed in some of the HA’s management areas, including pre-wetted salt and salt-treated using an Agricultural By-product (ABP). All the aforementioned materials contain chloride ions to a greater or lesser degree, which has a corrosive effect on elements of the highway infrastructure and also vehicles using the highway. The overall aim of the task is to gain a better understanding of the corrosive effects of these materials and treatment regimes. The task is divided into a number of separate experimental work items and this report details the methodology and results of the laboratory phase of the works.


2. Methodology

2.1 Test procedure

The laboratory test programme involved the accelerated corrosion testing of 102mm x 152mm rectangular metal samples of the following materials:

• Mild steel (Grade ASTM D609 Type 3 A366), • Galvanised steel (Hot dip galvanised ASTM D609 Type 3 A366), and • Aluminium (Grade 3003H14 alloy).

Although many different grades of these metal types exist and are in use, the above three basic metal types are all commonly used for highway infrastructure and vehicle construction applications. The testing was undertaken using a Cyclic Corrosion Test generally in accordance with ISO 14993 Corrosion of metals and alloys – Accelerated testing involving cyclic exposure to salt mist, “dry” and “wet” conditions, which comprises the following test cycle:

• 2 hrs continuous indirect spray with the test solution at 350C • 4 hrs air drying at 600C • 2 hrs condensing water climate at 500C

The two solutions tested were as follows:

• 50 gramme/litre (g/l) aqueous solution made from Thawrox 6, a commercially available dry salt used extensively by the HA’s Service Providers and in accordance with BS 3247 Fine Grade 6.3mm,

• 50 g/l aqueous solution made from Thawrox Plus which comprises Thawrox 6 plus 3% by weight Safecote ABP additive.

The concentration of test solution was selected solely on the basis that 50 g/l is commonly used in accelerated corrosion testing, and there is no relationship to actual de-icing practices where materials are on the whole spread in solid form and go into solution and then dilute and concentrate/crystallise depending on the prevailing environmental conditions. Each test was run for 63 cycles (21 days). This time period was initially selected based on the available financial budget, but as the testing proceeded a watch was also kept on the samples to ensure reasonable levels of corrosion were being obtained overall. In addition to the use of specific test solutions, as required for the purposes of this investigation, the only other deviation from the standard ISO test procedure was that one half of the samples were removed twice weekly and rinsed with tap water to remove any loose deposits/de-icer residue, with the aim of simulating the washing down (either natural or intentional) which is likely to occur in practice.


Prior to testing, all the metal samples were uniquely labelled and weighed. At weekly intervals during each test run one third of the samples were removed from the test chamber, photographed, the corrosion products removed, and then reweighed to determine the weight loss due to corrosion. Removal of corrosion products and measurement of corrosion weight loss was by a combination of physical and chemical methods in accordance with ISO 8407 Corrosion of metals and alloys – Removal of corrosion products from corrosion test specimens. The full matrix of samples tested is given in Table 2-1 below. Table 2-1 Laboratory testing matrix (no. samples)

Mild steel Galvanised steel Aluminium No

cycles Non-rinsed

Rinsed Non-rinsed

Rinsed Non-rinsed

Rinsed Total

50g/l Thawrox 6 7 3 3 3 3 3 3 14 3 3 3 3 3 3 21 3 3 3 3 3 3

54

5% Thawrox + 7 3 3 3 3 3 3 14 3 3 3 3 3 3 21 3 3 3 3 3 3

54

Total 36 36 36 108

2.2 Limitations

A limitation of the test programme is that it requires each de-icing agent to be already in solution. As a result it is unable to take into account the differences in relative corrosivity resulting from the differences in moisture content at the time of application and for this reason it has not been possible to compare the effects of dry salt with pre-wetted. In essence the validity of the data is therefore limited to evaluating whether the ABP additive has any significant direct corrosion inhibitive effect on chloride induced corrosion, all other factors being equal.


3. Results

The test data is presented in full in Appendix A with photographs of a representative selection of the samples given in Appendix B. A summary of the results is given below.

3.1 Aluminium

In all instances the percentage weight change of the aluminium samples was very small fluctuating around the zero value (Figure 3-1), suggesting that the material is resistant to chloride induced corrosion. There was no appreciable difference between those samples exposed to solutions of Thawrox Plus and Thawrox 6mm, with and without periodic rinsing.

-0.015%

-0.010%

-0.005%

0.000%

0.005%

0.010%

0.015%

0.020%

0.025%

0.030%

7 14 21

Days

Wei

ght c

hang

e (%

)

Thawrox 6 - Rinse

Thawrox 6 - No-rinse

Thawrox plus - Rinse

Thawrox plus - No-rinse

Figure 3-1 Average weight change of aluminium test samples


3.2 Mild steel

The corrosion data for both test solutions showed a general linear increase in weight loss with time, with over 10% of the sample weight being lost per week.

-40.0%

-35.0%

-30.0%

-25.0%

-20.0%

-15.0%

-10.0%

-5.0%

0.0%

7 14 21Days

Wei

ght c

hang

e (%

)

Thawrox 6 - Rinse




Figure 3-2 Average weight change of mild steel test samples The differences between the two test solutions, and also between the rinsed and non-rinsed samples were small, but it can be concluded that the solution of Thawrox Plus gave no appreciable reduction in the amount of metal loss over the period of the test programme when compared to the solution of Thawrox 6 (Figure 3-2). The 21 day samples suggest the opposite may even be the case when samples are exposed for longer periods of time, however, the magnitude of any difference that does exist is small compared to the overall corrosion level and well within the likely levels of experimental variability inherent in the test methodology (Figure 3-3).


-45.0%

-40.0%

-35.0%

-30.0%

-25.0%

-20.0%

-15.0%

-10.0%

-5.0%

0.0%7 14 21

Days

Wei

ght c

hang

e (%

)

Thawrox 6 - Rinse




Figure 3-3 Comparison of weight changes for mild steel test samples with and without periodic rinsing

3.3 Galvanised steel

The weight loss data for the galvanised steel samples indicated that the Thawrox Plus performed significantly better than the Thawrox 6mm after 7 days exposure. However, by 14 days the difference had closed significantly and by 21 days there was no discernable difference (Figure 3-4). There were also no discernable trends differentiating between the rinsed and non-rinsed specimens.


-6.0%

-5.0%

-4.0%

-3.0%

-2.0%

-1.0%

0.0%

7 14 21

Days

Wei

ght C

hang

e (%

)

Thawrox 6 - RinseThawrox 6 - No-rinseThawrox plus - RinseThawrox plus - No-rinse

Figure 3-4 Average weight change of galvanised mild steel test samples The test laboratory that carried out the experimental work initially reported significant difficulty in removing the zinc corrosion products and had expressed concern that some un-corroded zinc had been removed on the Thawrox 6mm 7 day samples as they were the first batch to be analysed. There was therefore concern that the apparent good performance by the Thawrox Plus at 7 days could have been a result of analytical error. In subsequent analysis batches (Thawrox 6mm, 14 and 21 day and all Thawrox Plus samples) the procedures were modified to address this concern and the Thawrox 6mm 7 day tests were repeated. The repeated tests gave significantly less scatter in the raw data than the initial batch, suggesting the laboratory’s concerns were founded, but the averages were remarkably similar (Figure 3-5). As a result it is believed that the data presented is robust and the changeable behaviour seen was a real phenomenon, and not a result of analytical error.


-8.00%

-7.00%

-6.00%

-5.00%

-4.00%

-3.00%

-2.00%

-1.00%

0.00%First run - rinse First run - no rinse Re-run - rinse Re-run - No rinse

Wei

ght c

hang

e (%

)

Figure 3-5 Comparison between initial and re-run for 7 day Thawrox 6mm galvanised steel samples


4. Discussion

From the data it is clear that neither test solution was able to cause measurable corrosion to the aluminium test samples in the duration of the test programme. As significant amounts of corrosion had occurred on the mild steel samples, it can be concluded that this lack of corrosion is due to the inherent corrosion resistance of the material and not that the test was run for an insufficient time. It is therefore suggested that the use of either dry, pre-wetted or ABP-treated salt should result in no discernable difference in the amount of corrosion currently seen on aluminium-based vehicles / highway assets. The data also indicated that, although significant in extent, there was no practical difference in the amount of corrosion on the mild steel samples exposed to the two test solutions. This indicates that, at the concentration present in the 50g/l test solution, the ABP additive is unable to reduce the level of corrosion by an appreciable amount. The data from the galvanised steel panels indicated that although initially the solution of Thawrox Plus was able to reduce the amount of corrosion over the solution of Thawrox 6mm, longer term there was also no discernable practical difference apparent. It is not clear why this behaviour was seen. Overall, therefore it would appear that the ABP additive has had no appreciable beneficial effect on the corrosion rate of the three metal types tested using the methodology adopted. It should be noted, however, that given the limitations inherent with any accelerated laboratory based investigation differences may still be seen between the various de-icing agents when used out on the network. If this proves to be the case then it would suggest they are either due to the increased concentration of the ABP additive at the point of application than that used in this test programme and/or differences in the winter maintenance procedures associated with their use. As a result any such differences that do manifest themselves are likely to be most apparent local to the spreading mechanism on the winter maintenance vehicle and are likely to dissipate with increasing distance.


5. Conclusions

Based on the results of the laboratory test programme, there is no indication that the ABP additive applied to rock salt has any active component present in sufficient amounts at the concentrations tested which is consistently and significantly able to reduce corrosion. Any benefit which is subsequently seen in practice through the use of either dry, pre-wetted or ABP-treated salt is therefore most likely a result of differences in moisture content at time of application, different spread rates and/or, in the case of the ABP treated salt, the significantly higher concentration of the ABP additive present at the point of application (than that used in this test programme). Such differences are therefore likely to only be seen very local to the spreading mechanism, dissipating rapidly with increasing distance from the rear of the winter maintenance vehicle.

Task Reference: 628 Page A-1 Laboratory Test Report

Appendix A – Experimental test results

Task Reference: 628 Page A-2 Laboratory Test Report

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Task Reference: 628 Page B-1 Laboratory Test Report

Appendix B – Photograph of test samples


B.1 Aluminium, Thawrox 6, Rinsed

Unexposed

7days

14days

21days


B.2 Aluminium, Thawrox 6, Unrinsed

Unexposed

7days

14days

21days


B.3 Aluminium, Thawrox Plus, Rinsed

Unexposed

7days

14days

21days


B.4 Aluminium, Thawrox Plus, Unrinsed

Unexposed

7days

14days

21days


B.5 Mild steel, Thawrox 6, Rinsed

Unexposed

7days

14days

21days


B.6 Mild steel, Thawrox 6, Unrinsed

Unexposed

7days

14days

21days


B.7 Mild steel, Thawrox Plus, Rinsed

Unexposed

7days

14days

21days


B.8 Mild steel, Thawrox Plus, Unrinsed

Unexposed

7days

14days

21days


B.9 Galvanised steel, Thawrox 6, Rinsed

Unexposed

7days

14days

21days


B.10 Galvanised steel, Thawrox 6, Unrinsed

Unexposed

7days

14days

21days


B.11 Galvanised steel, Thawrox Plus, Rinsed

Unexposed

7days

14days

21days


B.12 Galvanised steel, Thawrox Plus, Unrinsed

Unexposed

7days

14days

21days

de-icing materials and corrosion – laboratory testing report · corrosion of metals and alloys...

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