Experimental Multiscale Methodology for Predicting Material Fouling Resistance

Summary

Fouling, or corrosion product deposition, plagues numerous fields of energy production. At its core is adhesion between dissimilar materials. Understanding and controlling adhesion could reduce or eliminate fouling. This paper demonstrates pool boiling experiments (macroscale) and atomic force microscope force-spectroscopy measurements (microscale), which when in agreement, indicate fouling-resistance.

Abstract

The buildup of fouling deposits in energy systems degrades their ability to transfer heat, reducing efficiency and causing operational issues such as localized corrosion. Each application presents its own challenges. In nuclear power generation, the buildup of CRUD (Chalk River Unidentified Deposits) induces unique, negative effects ranging from reduced heat transfer, to axial power shifting, to CRUD-induced localized corrosion (CILC), the last of which can cause sudden fuel failure. Knowledge of why CRUD forms and how to prevent its adhesion to nuclear fuel rods represents a major step towards eliminating it. This paper demonstrates a methodology to ascertain which materials may resist CRUD adhesion, thereby preventing its initiation and growth. It presents experiments targeted at multiple length scales: pool boiling (macroscale), which simulates CRUD initiation processes in a nuclear reactor, and atomic force microscope (AFM) force spectroscopy (microscale), which yields quantitative measurements of the adhesion between CRUD constituents and candidate CRUD-resistant materials. Early data from both sets of experiments show some correlation, suggesting that AFM force spectroscopy (AFM-FS) may be used to pre-screen materials for fouling resistance. So far, it appears that CRUD constituents adhere well to oxides, while carbides show both greatly reduced adhesion and no adherent CRUD in pool boiling experiments. The relationship between the AFM-FS measurements and the growth of CRUD in the pool boiling experiments may provide insight into the possible factors affecting fouling in nuclear reactors, eventually leading to its elimination.

Introduction

Fouling, or the unwanted buildup of corrosion products on a surface, complements corrosion as a surface-based material degradation mechanism. Fouling is particularly detrimental in energy systems, as the surfaces which transfer the most enthalpy are typically the most susceptible. In essence, the presence of fouling deposits, often porous and/or thermally insulating, impedes the efficient flow of heat through a system. The high flow rates and large temperature differences in the hot and cold sections of a heat transfer system or coolant loop provide ideal environments to induce flow-assisted corrosion, and subsequent transport of corrosion products from areas producin....

Protocol

1. Material Preparation

1. Using a suitable machining method, section 1 cm x 1 cm coupons of the base substrate material (here, Zircaloy-4) from a sheet approximately 1 mm thick. The exact thickness is not important.
2. Using a manual or automatic polishing machine, polish the base substrates to an optically smooth, mirror finish.
   1. Mount specimens to a flat sample holder using a hot plate and a shear-resistant wax, for insertion into an automatic grinding/polishing machine. Here, mount twenty a.......

Discussion

This facility has several features that will produce reliable and useful results. First, the vertical orientation of the sample surface prevents solution precipitates from depositing on the surface, as was observed in Figures 5-6. Particularly important are the heat flux sensors, used to directly measure heat flux by differences in thermocouple height vs. temperature, rather than back-calculating a heat flux just from temperature measurements. This helps to further ensure experimental repeatability. Fina.......

Materials

Name	Company	Catalog Number	Comments
Atomic Force Microscope	Asylum	MFP3D-BIO
AFM Functionalized Cantilevers	Novascan		1 micron spheres glued on using proprietary technique
NiO Nanoparticles	Alfa Aesar		8-20nm average particle size
Fe3O4 Nanoparticles	Alfa Aesar		20-30mn average particle size
Silver conductive grease with silicone base	Chemtronics		The higher the thernal conductivity, the better
H3BO3	Mallinckrodt		99.945% pure
LiOH	Alfa Aesar		99.995% pure
Heat Flux Sensor (500kW/m2)	Captec Enterprise		1cm x 1cm
PID Temperature Controller	Omega	C9000A	Configured for SSR operation
Logomatic V2 Serial Data Logger in Enclosure	Ocean Controls		Acquires 0-3.3V, 0-12V, or 4-20mA signals on 8 channels at up to 1kHz