Introduction to NitCal as Sustainable Corrosion Inhibitor. Mehrdad Torabzadegan Yara Technology Centre, Porsgrunn, Norway

Transcription

1 Introduction to NitCal as Sustainable Corrosion Inhibitor Mehrdad Torabzadegan Yara Technology Centre, Porsgrunn, Norway

2 What is NitCal? NitCal is a Calcium Nitrate based concrete admixture. It is a chloride free, environment friendly admixture. Calcium Nitrate can be used: Stand alone product Setting accelerator Corrosion inhibitor Joint formulation with other chemical admixtures to have synergies with: Super plasticizers Blended cements Air entrainers Very good performance ready mix concrete precast concrete Wide range of environment casting temperature. O N O O Ca O N O O 2

3 Introduction to chloride ingress Concrete is a porous medium in which the pores are partially filled with water. Durability of the concrete directly linked to the ease of chemical penetration into the concrete. Chloride ion can migrate to the concrete by absorption and the diffuse further into the concrete through pore water. Concentration gradient is the main drive to migrate from surface to subsurface layer. When concrete permanently exposed to diffusion is the means of transport. However, even without drying the level of chloride ion in the pore water might be above the chloride source itself. 3

4 Findings: King Fahd University, KSA (2003) Study to evaluate commercially available corrosion inhibitors Calcium Nitrite and Calcium Nitrate (in different dosages) have been tested as corrosion inhibitors for reinforcement steel in concrete with different contaminations Corrosion potential measurement with electrodes Calcium Nitrate at dosage of 4% has increased compressive strength compare to other inhibitor. Calcium Nitrite and Calcium Nitrate perform similarly, and improve the situation especially for brackish water exposure or in case of contaminated aggregates Medium/Ion concentrations Sea Water Brackish water Chloride ppm 893 ppm Sulfate 4211 ppm 630 ppm Sodium ppm 459 ppm ph 7,6 7,78 corrosion potential [-mv] (SCE) high probability low probability Chloride Chloride & Sulfate Seawater Brackish water Unwashed aggregate ASTM C 8 76 limit 4

5 Findings: Saudi Aramco & King Fahd University, Saudi Arabia Study to evaluate commercially available corrosion inhibitors for silica fume modified concrete Among others Calcium Nitrite and Calcium Nitrate have been tested as corrosion inhibitors for reinforcement steel in concrete with different chloride contaminations Calcium Nitrite and Calcium Nitrate perform similarly, and the corrosion current in case of Nitrate was lower compared to Nitrite Ca(NO 3 ) 2 Ca(NO 2 ) 2 reference Refrence rebar with 2% chloride Rebar incorporating CN with 2% chloride CI-1: Calcium Nitrate CI-2: Calcium Nitrite Cl-3: Migratory corrosion inhibitor CI-4: Organic inhibitor Rebar incorporating MCI with 2% chloride Rebar incorporating CNI with 2% chloride 5

6 Findings: SINTEF long term effect of NitCal Wall elements exposed to sea water for 12 years The compressive strength of the concrete with nitrite was higher than concrete with nitrate and higher than the reference without admixture the compressive strength of both concrete with nitrite and reference concrete seemed to have a significant drop in strength from 1 year to 12 years, while the strength of concrete with nitrate was rather constant in the same period. The binder near the surface was substantially cracked in the concrete with nitrite, and the sulphate ingress was higher than for the two other concretes. Element 1 added nitrite Element 2 added nitrate Element 3 Reference Back Scattered Electron (BSE) images near the surface of the different mixes Compressive strength (MPa) Compressive strength of specimen 1 12 Year Nitrite Nitrate Ref 6

7 Findings: Fate of Nitrate ions in cement matrix Trace 3 Trace 3 Trace 2 Trace 2 Trace 1 Trace 1 CaCO3 free cement CaCO3 containing cement Trace 1: No Ca or Ca Trace 2: 0.01 moles Ca Trace 3: 0.01 moles Ca Cement hydration products: Portlandite Ca(OH) Calcium Silica Hydrate (C-S-H) Aluminium bearing phases: Aluminate Ferrite mono-substituent phase (AFm). Shorthand for a family of hydrated calcium aluminate phases. It has a crystalline layer structure and is derived from that of portlandite, Ca(OH), but with one third of the Ca ions replaced by a trivalent ion, nominally Al or Fe. Aluminate Ferrite tri-substituent phase (AFt) or ettringite. Product of C3A and gypsum and it has a trigonal needle shaped crystalline. 7

8 Findings: Fate of Nitrate ions in cement matrix Sulphite to aluminate ratio (SO3/Al2O3) in cement type I/II is 0,5 while in cement type II/V is 0,83. Before adding nitrate XRD graph of cement types I/II shows carbonate and hemi-carbonate AFm in addition to un hydrated C3S. After adding Nitrate, carbonate and hemi-carbonate mostly are replaced by nitrate. Before adding nitrate XRD graph of cement types II/V shows sulphate bonding to AFm in addition to situation in Cement type I/II. After adding Nitrate,, or OH ions are expelled from the AFm phase at the expense of the nitrate-afm (NO3-AFm) formation. 8

9 Influence of nitrate and nitrite on AFm phase Characterization - Scanning Electron Microscopy (SEM) Both crystals look similar Significant wrapped and leaf like edges. Nitrite AFm Nitrate AFm 9

10 Solid Chemistry: Nitrate activated at the presence of Chloride Nitrate is bound to AFmphases during cement hydration Due to the different binding powers (= Gibbs Energies for the solid system) Chloride can replace Nitrate/Nitrite in the matrix Thus bound Nitrate is released and available for hydroxide formation at the presence of chloride; chloride gets bound! Balonis et al. (2010) 10

11 Aqueous Chemistry: The formation of Ferric Hydroxide layers Does not happen in concrete (no self passivation) Does happen! Does happen! NO2- mechanism: Rosenberg and Gaidis (1979), Sagoe-Crentsil et al (1991/1992) NO3- mechanism: Justnes (2005) 11

12 Introduction to ASTM Standard C1582/C1582M-11 Scope Standard specification for admixtures to inhibit chloride-induced corrosion of reinforced steel in concrete. The specification covers the material for use as chloride-corrosioninhibiting admixtures for concrete. Terminology: Chloride ion content: The acid-soluble chloride-ion content measured at the depth of reinforcement. Critical chloride-ion content: The mean chloride ion content in the unreinforced control beams at. is time to the mean integrated macro-cell current of the control beams reaches 50 C. is time to the mean integrated macro-cell current of the control beams reaches 150 C. Completion of testing when following conditions reached: Mean integrated macro-cell current in control beam 150 C. Mean chloride ion content Critical ion content. 12

13 Requirements according to ASTM C1582/C1582M General Requirements shall be conform to the requirements prescribed in Table 1. The performance property of chloridecorrosion inhibiting shall consist of time of setting and compressive strength at 3, 7, and 28 days. Remaining compressive and flexural strength of 80% of the reference sample. At any age compare to the age before the reduction should not be more than 10%. Initial and final setting time should not be altered more than 3.5 hours. The chloride-corrosion inhibitor shall comply with ASTM C494/C494M specification as admixture. Aggregate origin 13

14 Requirements according to ASTM C1582/C1582M Corrosion-Inhibiting Performance ASTM Test Method G109 or G180 Corrosion potential requirement (according to Table 2) Corrosion current measurement Corroded area measurement Chloride ion content Scope: G109 method covers a procedure for determining the effect of chemical admixtures on the corrosion of the concrete (chapter 1 ASTM G109) Composition requirements by ASTM G109: CEM I 42,5R (EN 197/1) W/C 0,5 Air entrainer 6±1% Ponding cycle: 15 days dry 15 days of ponding (3% NaCl solution) 14

15 Test Setup - Concrete beam Measure the voltage across the 100 Ω resistor Measure the corrosion potential of bars against a ref electrode placed in salt containing dam. Plastic tube + tape Stainless steel screw 20 mm Plastic tube +tape

16 Test Results according to test method G109 setting time According to ASTM requirements: Time of setting, allowable deviation from control Initial: not more than 3:30 earlier or later Final: not more than 3:30 earlier or later Setting time Dosage 0% CN Dosage 4% CN T Criteria (hh:mm) Initial End Initial End Initial End T < +/- Initial End CEM I 52.5 R 04:30 06:15 02:10 03:05 02:20 03:10 03:30 fulfilled fulfilled CEM IV/A 42.5R 06:00 07:25 03:50 04:50 02:10 02:35 03:30 fulfilled fulfilled CEM II/A-LL 42.5R 04:30 05:55 02:30 03:35 02:00 02:20 03:30 fulfilled fulfilled Results from ENCO report, Italy Setting time (hh:mm) 0% CN 4% CN T T < +/- Criteria CEM I 52.5 R white 03:00 01:00 02:00 03:30 fulfilled CEM II/A-V :30 02:00 02:30 03:30 fulfilled CEM III/B :00 00:30 03:30 03:30 fulfilled Results from various reports by SINTEF, Norway 16

17 Test Results according to test method G109 Compressive and flexural strength According to ASTM requirements: Compressive strength and flexural strength minimum remain 80% of control 1 day strength Dosage 0% CN Dosage 4% CN remain _c [MPa] _f [MPa] _c [MPa] _f [MPa] _c _f CEM I 52.5 R 34,50 5,70 36,00 5, % 98 % CEM II/A-V ,20 3,70 14,30 3,10 83 % 84 % Results from various reports by SINTEF, Norway, over the last decades CEM III/B ,50 3,00 12,20 2,70 84 % 90 % compressive strength (MPa) Compressive strength (CEM I 52.5 R-LA) 0 40% time of experiment (days) 140% 120% 100% 80% 60% remaining strength Dosage 0% CN s_c [MPa] Dosage 2% CN s_c [MPa] remain (%) s_c req. Remain (%) 17

18 Completion of testing (corrosion current) start and end first check point Corrosion Current starts at 10 μa (G109). Control reached 10 μa in average after 97 days. 3% CN treated samples reached current at 150 days and 4% CN samples not within 180 days. corrosion curent (μa) Corrosion Current Control (avr.) 3% CN (avr.) 4% CN (avr.) G109 untreated A untreated B untreated C 3% CN A 3% CN B 3% CN C 4% CN A 4% CN B 4% CN C duration of test (days) Results from ENCO (2014) 18

19 Completion of testing Start and end second check point Mean integrated macro-cell current in control beam 150 C. That happened in day 156 th, interpolated from the measurement at 150 days and 180 days Concrete beams with 3% and 4% of NitCal reached about same time to macro-cell charge of 50 C. 250 Period of testing % NitCal 4% NitCal Control mix Charges (C) Days Results from ENCO (2014) 19

20 Completion of testing Start and end third check point Mean chloride ion content Critical ion content Critical chloride ion content is the concentration that the corrosion is possible. Based on below table 0.4% of cement weight is determined. (Browne 1982) 0.50 Chloride content 0.45 chloride content (%) Adopted from (Browne 1982) Control (avr.) 3% CN (avr.) 4% CN (avr.) % Results from ENCO (2014) 20

21 Corroded area Corrosion inhibition performance Corroded area of the treated samples shall not exceed 1/3 of the corroded area of the reference samples (C1582) After 180 days the corroded area was in average about 1/3 of the reference samples. (3% and 4% CN dosage) 4.0 Corroded area (180d) absolute relative absolute area in (cm2) relative area (%) A B C avr(a,b,c) A B C avr(a,b,c) A B C avr(a,b,c) ASTM C1582 untreated 3% CN 4% CN Req Results from ENCO (2014) 21

22 Corroded area Corrosion inhibition performance 3% NitCal Extracted bars from concrete test beams confirm less corroded area for beam with 3% NitCal to the sample with 4%. Reference 0% NitCal 4% NitCal Results from ENCO (2014) 22

23 Why choose Nitrate and not Nitrite Chemically: Calcium Nitrate Ca(NO3)2 contains one Oxygen atom more than Calcium Nitrite Ca(NO2)2 therefore more available to form passive ferric hydroxide layers. Health and Safety: Nitrate is no hazardous material (e.g. utilized as fertilizer and energy source for micro-organisms), whereas Nitrite is poisonous (biocide and conservation agent) O N O O Ca O N O O Economy: Depending on market and logistics, Nitrate is significantly cheaper than Nitrite 23

24 Summary With reference to literatures: NitCal meets the general requirements of ASTM C1582 regarding setting time and compressive strength. Setting time is usually altered less than 3,5 hours. Compressive strength is usually > 80% of the reference at all times. With reference to tests elaborated at ENCO: NitCal, added as 3% and 4% bwoc. with minor deviation matches the corrosion inhibitor requirements of ASTM C1582 (when tested in accordance with test method ASTM G109). The corrosion current stays within the documented tests below 10 µa. The integrated charges stay below 50 C (for 150 days). The corroded surface is about 1/3 of the reference (180 days). Except lower corrosion current at 150 and 180 days, no significant improvement was found in the performance of calcium nitrate when the percentage was increased from 3% to 4%. 24

25 IMPORTANT NOTICE Yara disclaims all responsibility and liability for any expenses, losses, damages and costs incurred as a result of relying on or using the information contained in this document. Yara reserves the right to adjust and revise this document at any time. Any forward-looking statement made by Yara in this document is based only on information currently available to Yara and speaks only as of the date on which it is made. Yara undertakes no obligation to publicly update any forward-looking statement. No rights, including, but not limited to, intellectual property rights, in respect of this document are granted to any recipient unless specifically stated. Yara International ASA. All rights reserved. Yara Technology Centre, Porsgrunn, Norway 22/12/

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