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Title: Protective coatings for alloys in contact with molten drawsalt (NaNO/sub 3/-KNO/sub 3/)

Abstract

Molten drawsalt (NaNO/sub 3/-KNO/sub 3/) is being considered as the energy transfer and storage medium for many solar central receiver applications. In an effort to reduce the cost of the containment material while maintaining corrosion resistance, alloys with aluminide coatings have been examined while in contact with molten drawsalt for more than 6000 hours at 600/sup 0/C. The alloys examined were 2-1/4 Cr-1 Mo, 5 Cr-1/2 Mo, and 9 Cr-1 Mo low-alloy steels, and 316 stainless steel. The results show a steady, albeit slow, net weight loss over the course of the experiment. The weight loss has been attributed to spalling of Al/sub 2/O/sub 3/ from the surface (the occurrence of Al/sub 2/O/sub 3/ is a result of the aluminizing process) and dissolution of corrosion products NaAlO/sub 2/ and/or NaFeO/sub 2/ during post-immersion handling. Scanning electron micrographs of exposed surfaces revealed little or no corrosion of the base metal. It has been concluded that aluminide coated alloys could provide significant cost savings (approx. 50%) relative to Incoloy 800, and provide at least equivalent corrosion resistance.

Authors:
; ;
Publication Date:
Research Org.:
Sandia National Labs., Livermore, CA (USA)
OSTI Identifier:
6741849
Alternate Identifier(s):
OSTI ID: 6741849; Legacy ID: DE83002505
Report Number(s):
SAND-82-8028
ON: DE83002505
DOE Contract Number:
AC04-76DP00789
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; CHROMIUM-MOLYBDENUM STEELS; CORROSION PROTECTION; PROTECTIVE COATINGS; MOLTEN SALTS; CORROSIVE EFFECTS; POTASSIUM NITRATES; SODIUM NITRATES; STAINLESS STEEL-316; TOWER FOCUS COLLECTORS; ALUMINIUM OXIDES; CORROSION PRODUCTS; EXPERIMENTAL DATA; HEAT STORAGE; HEAT TRANSFER FLUIDS; MATERIALS TESTING; PHASE DIAGRAMS; WEIGHT; ALKALI METAL COMPOUNDS; ALLOYS; ALUMINIUM COMPOUNDS; CHALCOGENIDES; CHROMIUM ALLOYS; CHROMIUM STEELS; CHROMIUM-NICKEL STEELS; COATINGS; CONCENTRATING COLLECTORS; CORROSION RESISTANT ALLOYS; DATA; DIAGRAMS; ENERGY STORAGE; EQUIPMENT; FLUIDS; HEAT RESISTANT MATERIALS; HEAT RESISTING ALLOYS; INFORMATION; IRON ALLOYS; IRON BASE ALLOYS; MATERIALS; MOLYBDENUM ALLOYS; NICKEL ALLOYS; NITRATES; NITROGEN COMPOUNDS; NUMERICAL DATA; OXIDES; OXYGEN COMPOUNDS; POTASSIUM COMPOUNDS; SALTS; SODIUM COMPOUNDS; SOLAR COLLECTORS; SOLAR EQUIPMENT; STAINLESS STEELS; STEELS; STORAGE; TESTING 140702* -- Solar Thermal Power Systems-- Central Receiver; 360105 -- Metals & Alloys-- Corrosion & Erosion

Citation Formats

Carling, R.W., Bradshaw, R.W., and Mar, R.W. Protective coatings for alloys in contact with molten drawsalt (NaNO/sub 3/-KNO/sub 3/). United States: N. p., 1982. Web. doi:10.2172/6741849.
Carling, R.W., Bradshaw, R.W., & Mar, R.W. Protective coatings for alloys in contact with molten drawsalt (NaNO/sub 3/-KNO/sub 3/). United States. doi:10.2172/6741849.
Carling, R.W., Bradshaw, R.W., and Mar, R.W. Wed . "Protective coatings for alloys in contact with molten drawsalt (NaNO/sub 3/-KNO/sub 3/)". United States. doi:10.2172/6741849. https://www.osti.gov/servlets/purl/6741849.
@article{osti_6741849,
title = {Protective coatings for alloys in contact with molten drawsalt (NaNO/sub 3/-KNO/sub 3/)},
author = {Carling, R.W. and Bradshaw, R.W. and Mar, R.W.},
abstractNote = {Molten drawsalt (NaNO/sub 3/-KNO/sub 3/) is being considered as the energy transfer and storage medium for many solar central receiver applications. In an effort to reduce the cost of the containment material while maintaining corrosion resistance, alloys with aluminide coatings have been examined while in contact with molten drawsalt for more than 6000 hours at 600/sup 0/C. The alloys examined were 2-1/4 Cr-1 Mo, 5 Cr-1/2 Mo, and 9 Cr-1 Mo low-alloy steels, and 316 stainless steel. The results show a steady, albeit slow, net weight loss over the course of the experiment. The weight loss has been attributed to spalling of Al/sub 2/O/sub 3/ from the surface (the occurrence of Al/sub 2/O/sub 3/ is a result of the aluminizing process) and dissolution of corrosion products NaAlO/sub 2/ and/or NaFeO/sub 2/ during post-immersion handling. Scanning electron micrographs of exposed surfaces revealed little or no corrosion of the base metal. It has been concluded that aluminide coated alloys could provide significant cost savings (approx. 50%) relative to Incoloy 800, and provide at least equivalent corrosion resistance.},
doi = {10.2172/6741849},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Sep 01 00:00:00 EDT 1982},
month = {Wed Sep 01 00:00:00 EDT 1982}
}

Technical Report:

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  • Molten drawsalt (NaNO/sub 3/-KNO/sub 3/) is being considered as the energy transfer and storage medium for many solar central receiver applications. In an effort to reduce the cost of the containment material while maintaining corrosion resistance, alloys with aluminide coatings have been examined while in contact with molten drawsalt for more than 6000 hours at 600 /sup 0/C. The alloys examined were 2.25 Cr-1 Mo, 5 Cr-0.5 Mo, and 9 Cr-1 Mo low-alloy steels, and 316 stainless steel. The results show a steady, albeit slow, net weight loss over the course of the experiment. The weight loss has been attributedmore » to spalling of Al/sub 2/O/sub 3/ from the surface (the occurrence of Al/sub 2/O/sub 3/ is a result of the aluminizing process and dissolution of corrosion products NaAlO/sub 2/ and/or NaFeO/sub 2/ during post-immersion handling. Scanning electron micrographs of exposed surfaces revealed little or no corrosion of the base metal. It has been concluded that aluminide coated alloys could provide significant cost savings (about 50 pct) relative to INCOLOY Alloy 800, and provide at least equivalent corrosion resistance.« less
  • The corrosion of Fe-Ni-Cr alloys by draw salt (60 wt % NaNO/sub 3/-40 wt % KNO/sub 3/) is studied with thermal convection loops of alloy 800 and types 304L and 316 stainless steel. The main corrosion processes at 600/sup 0/C and below were the growth of thin oxide scales and the dissolution of chromium by the salt. Spallation of oxide layers occurred on type 304 stainless steel specimens at intermediate temperatures. Results indicated relatively low corrosion rates (< 13 ..mu..m/year in most cases) for temperatures of 600/sup 0/C and less. Corrosion of type 316 stainless steel was greatly accelerated whenmore » the maximum loop temperature was raised to 620/sup 0/C. It therefore appears that 600/sup 0/C may be the limiting temperature for use of the above alloys in draw salt.« less
  • Boiling temperature measurements have been made for saturated ternary solutions of NaCl + KNO{sub 3} + H{sub 2}O and NaNO{sub 3} + KNO{sub 3} + H{sub 2}O over the full solute mole fraction range, along with the limiting binary solutions NaCl + H{sub 2}O, NaNO{sub 3} + H{sub 2}O, and KNO{sub 3} + H{sub 2}O. Boiling temperatures have also been measured for the quaternary NaCl + NaNO{sub 3} + KNO{sub 3} + H{sub 2}O mixtures with KNO{sub 3}:NaNO{sub 3} mole ratios of 1.01 and 1.19, which corresponding to the eutectic ratio and a near-eutectic ratio for the NaNO{sub 3} +more » KNO{sub 3} + H{sub 2}O subsystem. The maximum boiling temperature found for the NaCl + KNO{sub 3} + H{sub 2}O system is 134 C and for the NaNO{sub 3} + KNO{sub 3} + H{sub 2}O system is 160 C, but boiling temperatures as high as 196 C were measured the NaCl + NaNO{sub 3} + KNO{sub 3} + H{sub 2}O system. These mixture compositions correspond to the major mineral assemblages that are predicted to control the deliquescence relative humidity of salts found by leaching dust samples from the proposed nuclear repository at Yucca Mountain, Nevada.« less
  • The corrosion behavior of Type 304 stainless steel in molten NaNO/sub 3/-KNO/sub 3/ was studied at temperatures between 600/sup 0/C and 350/sup 0/C using thermal convection loops. Corrosion rates were somewhat less than 2.5 x 10/sup -2/ mm/year (1 mil/year) at the maximum temperature. Two corrosion processes were observed, formation of oxide scales and depletion of chromium from the alloy. Oxidation products generally consisted of at least two layers, a layer of Fe/sub 3/O/sub 4/ over an iron-chromium spinel. In addition, a complex oxide was detected which appeared to be a double oxide of iron and, a salt impurity, magnesium.more » Chromium accumulated as a soluble product in the melt but thermal gradient mass transfer was not observed. Chromium depletion kinetics were approximately parabolic with time suggesting a diffusion controlled process. Autogenous weldments experienced somewhat more corrosion in the heat-affected zone than either the fusion zone or the parent alloy.« less
  • The corrosion behavior of Alloy 800 and Type 316 stainless steel in molten NaNO/sub 3/-KNO/sub 3/ was studied at temperatures from 605/degree/C to 630/degree/C. Corrosion behavior was significantly different from that previously reported in nitrate melts at temperatures up to 600/degree/C and involved a combination of oxidation, internal nitridation and sodium metallate formation. Corrosion kinetics, determined metallographically, switched from a parabolic to a linear rate equation as temperature increased. Corrosion was uniform and resulted in metal losses on the order of 100 micronsyear at 630/degree/C. Among the alloying elements, chromium was depleted from the alloy as the result of amore » basic fluxing process. The kinetic equations describing chromium depletion also changed from parabolic to linear with increasing temperature. The effect of the equilibrium chemistry of the melt on the corrosion behavior of the alloys is analyzed and possible corrosion mechanisms are discussed. 24 refs., 8 figs., 2 tabs.« less