Prepared by:N. ColeNCC Engineering

M. DigheGeneral Electric

S. RangaswamiWall Colmonoy Corporation

ContentsIntroduction 146

Fundamentals 146

Types of Corrosion 147

Proper BrazingProcedures toMinimize Corrosion 160

Removing Sourceof Corrosion 161

Corrosion Resistance 163

Bibliography 166

SuggestedReading List 166

Photograph courtesy of D. Sigler, J. Schroth, and the General Motors Corporation

CORROSION OFBRAZED JOINTS

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AWS BRAZING HANDBOOK 145

INTRODUCTION

146 CHAPTER 7—CORROSION OF BRAZED JOINTS AWS BRAZING HANDBOOK

Corrosion is often thought of as rusting, the pro-cess of deterioration of metal when it is exposed toair or water. However, corrosion reactions are actu-ally much broader in scope. Whereas rusting involvesthe corrosion of iron or steel, other materials alsosuffer deterioration when they react with the envi-ronment. In a wider sense, corrosion can be definedas any reaction between a material, whether this be ametal, plastic, or ceramic, and its environment,resulting in a reduction in the material’s ability toperform the service for which it was intended.

This chapter addresses various forms of corrosionobserved in brazed joints, their causes, the role ofproper brazing procedures in controlling corrosion,and the corrosion resistance of various brazing fillermetal systems.

FUNDAMENTALS

Inasmuch as corrosion can cause degradation,structural failure, and equipment or plant downtimefor replacement or repair, a great deal of time andeconomic resources are spent to protect components.Corrosion occurs on practically all metals in a widevariety of environments. Reactions occur in liquidssuch as aqueous solutions; molten salts; molten met-als; and in air, oxygen, or other gaseous compoundsat ambient or elevated temperatures. The degrada-tion can be obvious or subtle. The structure may notactually be affected by corrosion, but a change inappearance may render the material unusable orcause contamination, particularly in food, drugs, orpaint.

The effects of diffusion and alloying with the basemetal during brazing (and during service) also influ-

ence compatibility. For this reason, each brazingfiller metal class must be considered in conjunctionwith the various base materials commonly brazedwith it. For specific applications, surveillance testingin actual or simulated systems should be performedin order to model actual service conditions.

The deterioration of materials is not limited tometals. As more brazed ceramics are used in varyingenvironments, more will be written about their cor-rosion behavior. Such ceramic-based materials arewidely used in automotive, aerospace and energyapplications. In many cases, ceramic-to-ceramic orceramic-to-metal brazing is required. Many ceramicsapplications include exposure to high-temperature cor-rosive environments. As a result, the high-temperaturecorrosion behavior of ceramic brazed joints is criti-cal. Ceramics, by nature, are highly corrosion- andoxidation-resistant materials. However, the brazingfiller metals often used to braze ceramics have infe-rior corrosion resistance. Many times, the brazingfiller metals form intermediate phases with theceramic or the matrix material (in case of compos-ites), altering the chemical composition locally. Thiscan affect corrosion in two ways. The new phaseformed may be much less corrosion resistant thanthe ceramic or the brazing filler metal, or the differ-ent chemistries may form a galvanic cell, increasingthe chances of corrosion. Galvanic corrosion isdiscussed in detail in the following section of thischapter.

Braze joint design can also greatly influence corro-sion response—often by means of mechanical effectssuch as complicating stress factors. Brazed joint per-formance is improved when the joint is designed sothat the braze, when stressed, is placed in shear orcompression rather than tension. The stress and cor-

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