PRECLEANING AND SURFACE PREPARATION

Reviewers

Y. FlomNASA Goddard Space Flight Center

D. S. PetersonModine Manufacturing Company

R. W. SmithMaterials Resources International

R. L. HoldrenEdison Welding Institute

Contents

Introduction 128

Cleaning Processes 128

Surface Pretreatments 133

Braze Flow Inhibitors 134

MaintainingCleanliness 134

SuggestedReading List 135

CHAPTER 5

Photograph courtesy of D-N-R, Incorporated

AWS BRAZING HANDBOOK 127

INTRODUCTION

128 CHAPTER 5—PRECLEANING AND SURFACE PREPARATION AWS BRAZING HANDBOOK

Clean surfaces are essential to the formation ofsound brazed joints of uniform quality. Capillaryattraction is enhanced when the various componentsto be brazed and the brazing filler metal are free ofgrease, oil, dirt, scale, and other solid or liquid con-taminants prior to brazing.

While brazing fluxes and atmospheres can pro-vide some cleaning action, this is not their primarypurpose. Thus, they should not be used as a substi-tute for precleaning operations. Fluxes and brazingatmospheres can be used to remove oxide surfacelayers, penetrate thin oxide surface layers, or preventthe formation of oxide surface layers on joint fayingsurfaces during preheating and brazing. Fluxes orbrazing atmospheres also reduce the surface tensionof molten brazing filler metals and can form protec-tive cover layers over solidifying brazing filler metalsin the brazed joints. However, the effectiveness offluxes and atmospheres in removing existing oxidesin addition to their primary functions may be inade-quate (see Chapter 4, “Fluxes and Atmospheres”).

The purposes of cleaning and surface pretreat-ments are many. However, these steps are mainlyperformed to clean organic residues, reduce theamount of oxide scaling, or prepare the faying sur-faces to better accept and react with the molten braz-ing filler metals during the brazing cycles.

The length of time that the surfaces remain cleanafter cleaning depends on the materials involved, theatmospheric conditions, the amount and method ofhandling undergone by the components to be brazed,and storage conditions, among other factors. How-ever, it is recommended that brazing be accomplishedas soon as possible after the material has beencleaned. Maintaining a flow of assemblies (i.e., firstin, first out) that minimizes time delay between thecleaning and brazing operation is essential, particu-

larly for atmosphere or vacuum brazing processes.Protection from contamination during storage isequally important. Maximum storage time and con-ditions must be determined for each product, andthese should be included in the brazing procedurespecification (BPS).

Preparing surfaces for brazing can be discussed intwo major classifications—cleaning and surfacetreatments. The term cleaning can be defined as ameans for removing from the faying surfaces con-tamination that would prevent brazing filler metalflow and wetting. The term surface treatments refersto procedures that modify or add to a surface toenhance wetting and the adherence of the moltenbrazing filler metals. The following sections examinevarious processes and their attributes.

CLEANING PROCESSES

Surface cleaning methods can involve chemical,mechanical, or physical cleaning as well as thermaltreatment. They are typically employed to removesubstances in the following order:

1. Pigmented drawing and forming compounds;2. Oils, greases, and waxes;3. Scales, oxides, and smut; and4. Abrasive and grinding residues.

Pigmented compounds as well as some cuttingand machining oils are difficult to remove. They mayrequire the use of multiple cleaning methods involv-ing solvents, hot emulsion sprays, hot alkaline soaks,and hot rinses. The elapsed time between the appli-cation of these compounds and the cleaning process

PRECLEANING AND SURFACE PREPARATION

CHAPTER 5