化学镀镍的表面处理
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193
Βιβλιοθήκη Baidu
194
ELECTROLESS PLATING
Another reason for the need of a very careful selection of the preparation process is that it can significantly affect the porosity of the metal deposit. This is particularly true for electroless nickel. Residues from cleaners and deoxidizers can increase the porosity of electroless nickel, creating passive spots that will not initiate electroless plating (6). Thin (less than 5 pm) electroless nickel deposits are more porous than electrodeposited nickel of comparable thickness. During electroless nickel plating, the deposition process initiates at discrete sites and the substrate will become fully covered through the lateral growth of these sites. Short plating times will not allow full coverage of the base material, causing porous electroless deposits.
IRON AND FERROUS ALLOYS
Iron and its alloys are the most frequently plated substrates. From a surface preparation standpoint, they can be grouped as low alloy and carbon steels; cast iron; and high alloy steels. The principles of preparing ferrous surfaces are based on successive steps for soil removal, deoxidation, and surface activation. The manner in which these steps are carried out will depend on the type of alloy processed. Steel and cast iron parts are cleaned using conventional techniques such as alkaline cleaning, solvent cleaning, or electrocleaning similar to the processes used in electroplating or other metal finishing procedures. In somecases, where the surface has been severely corroded or is covered by scale or other strongly adherent residues, mechanical pretreatments such as sandblasting, shot peening, or wheelabrading may be necessary. Alkaline soak cleaning with commercial cleaners are frequently preferred. These materials contain a combination of alkaline sodium compounds such as hydroxide, carbonate, silicates, phosphates, and organic surfactants. The selection of the cleaner will depend on the nature of the surface contamination and substrate, and the assistance of the supplier can bevery helpful in achieving effective and economical cleaning. In some cases, solvent or vapor degreasing may be used prior to the alkaline cleaning steps. A good summary of cleaning practices for steel can be found in the ASTM Standard Practices (5). Carbon and low-alloy steels are deoxidized using either acid pickling solutions or alkaline deoxidizers. Pickling solutions, in general, use hydrochloric or sulfuric acids, combinations of these mineral acids, or salts of the acids. On carbon steels, hydrochloric acid is used at 10 to 50 percent concentration while sulfuric acid (2 to 10 percent) is also used. Sulfamic acid pickles are recommended for leaded steels. It is convenient to avoid the use of pickling inhibitors in this process, since they may interfere in the initiation and activation necessary for electroless plating. Alkaline deoxidizers containing organic chelating agents and/or sodium cyanide are frequently used in surface preparation, since they offer specific advantages over acid deoxidizers. Alkaline deoxidants will remove oxides without attacking the substrate metal and are less prone to cause hydrogen
Chapter 7 Surface Preparation For Electroless Nickel Plating
Juan Hajdu
Electroless nickel is plated over a large number of metallic and nonmetallic substrates of very different compositions and properties. For these reasons, it is impossible to select a single general approach to surface preparation. Specific procedures are required for each type of substrate. In broad terms, we can classify the substrates as metallic and nonmetallic. Most electroless nickel plating is done on metal parts, and in this chapter we will discuss mainly the preparation of metal surfaces. An important characteristic of plating metals is the strength of the bond that can develop between the base metal and the coating. Metal-to-metal bonds with high adhesion values require thorough surface preparation-removing from the base metal surface foreign contaminants (soil, dirt, corrosion products, oxides, tarnish, and others), and eliminating mechanically distorted surface layers-to present a clean, healthy surface structure. The removal of foreign contaminants is generally accomplished by using commercial alkaline cleaners. The selection is based on the nature of the contaminants and the type of substrate. The suppliers of cleaners will assist in the selection of the right material; there are also numerous literature sources that discuss metal surface cleaning (1-5). The removal of surface oxidation and unwanted metal is accomplished by chemical attack. Acid pickling solutions and alkaline deoxidizing materials, similar to those used in electroplating, are also effective for electroless nickel plating. In some cases, mechanical surface treatments, such as shot peening or sandblasting, are used in surface finishing prior to chemical treatment, especially with large, expensive parts that allow the use of manual processing. Electroless plating differs from other metal coating techniques (electroplating, vacuum metallizing, hot dip galvanizing) in that the substrate initiates the autocatalytic chemical reduction process. Some of the metals, such as nickel, are catalytic when immersed in an electroless plating solution, while steel and aluminum become catalytic by the formation of an immersion nickel deposit in the electroless nickel bath. Other metals, such as copper, are passive and require activating steps in order to initiate the electroless nickel plating process. It should be noted that even naturally active surfaces can become passive when contaminated by foreign residues or oxide layers. For this reason, surface preparation for electroless nickel plating requires the highest degree of care and control of all metal finishing procedures.
Βιβλιοθήκη Baidu
194
ELECTROLESS PLATING
Another reason for the need of a very careful selection of the preparation process is that it can significantly affect the porosity of the metal deposit. This is particularly true for electroless nickel. Residues from cleaners and deoxidizers can increase the porosity of electroless nickel, creating passive spots that will not initiate electroless plating (6). Thin (less than 5 pm) electroless nickel deposits are more porous than electrodeposited nickel of comparable thickness. During electroless nickel plating, the deposition process initiates at discrete sites and the substrate will become fully covered through the lateral growth of these sites. Short plating times will not allow full coverage of the base material, causing porous electroless deposits.
IRON AND FERROUS ALLOYS
Iron and its alloys are the most frequently plated substrates. From a surface preparation standpoint, they can be grouped as low alloy and carbon steels; cast iron; and high alloy steels. The principles of preparing ferrous surfaces are based on successive steps for soil removal, deoxidation, and surface activation. The manner in which these steps are carried out will depend on the type of alloy processed. Steel and cast iron parts are cleaned using conventional techniques such as alkaline cleaning, solvent cleaning, or electrocleaning similar to the processes used in electroplating or other metal finishing procedures. In somecases, where the surface has been severely corroded or is covered by scale or other strongly adherent residues, mechanical pretreatments such as sandblasting, shot peening, or wheelabrading may be necessary. Alkaline soak cleaning with commercial cleaners are frequently preferred. These materials contain a combination of alkaline sodium compounds such as hydroxide, carbonate, silicates, phosphates, and organic surfactants. The selection of the cleaner will depend on the nature of the surface contamination and substrate, and the assistance of the supplier can bevery helpful in achieving effective and economical cleaning. In some cases, solvent or vapor degreasing may be used prior to the alkaline cleaning steps. A good summary of cleaning practices for steel can be found in the ASTM Standard Practices (5). Carbon and low-alloy steels are deoxidized using either acid pickling solutions or alkaline deoxidizers. Pickling solutions, in general, use hydrochloric or sulfuric acids, combinations of these mineral acids, or salts of the acids. On carbon steels, hydrochloric acid is used at 10 to 50 percent concentration while sulfuric acid (2 to 10 percent) is also used. Sulfamic acid pickles are recommended for leaded steels. It is convenient to avoid the use of pickling inhibitors in this process, since they may interfere in the initiation and activation necessary for electroless plating. Alkaline deoxidizers containing organic chelating agents and/or sodium cyanide are frequently used in surface preparation, since they offer specific advantages over acid deoxidizers. Alkaline deoxidants will remove oxides without attacking the substrate metal and are less prone to cause hydrogen
Chapter 7 Surface Preparation For Electroless Nickel Plating
Juan Hajdu
Electroless nickel is plated over a large number of metallic and nonmetallic substrates of very different compositions and properties. For these reasons, it is impossible to select a single general approach to surface preparation. Specific procedures are required for each type of substrate. In broad terms, we can classify the substrates as metallic and nonmetallic. Most electroless nickel plating is done on metal parts, and in this chapter we will discuss mainly the preparation of metal surfaces. An important characteristic of plating metals is the strength of the bond that can develop between the base metal and the coating. Metal-to-metal bonds with high adhesion values require thorough surface preparation-removing from the base metal surface foreign contaminants (soil, dirt, corrosion products, oxides, tarnish, and others), and eliminating mechanically distorted surface layers-to present a clean, healthy surface structure. The removal of foreign contaminants is generally accomplished by using commercial alkaline cleaners. The selection is based on the nature of the contaminants and the type of substrate. The suppliers of cleaners will assist in the selection of the right material; there are also numerous literature sources that discuss metal surface cleaning (1-5). The removal of surface oxidation and unwanted metal is accomplished by chemical attack. Acid pickling solutions and alkaline deoxidizing materials, similar to those used in electroplating, are also effective for electroless nickel plating. In some cases, mechanical surface treatments, such as shot peening or sandblasting, are used in surface finishing prior to chemical treatment, especially with large, expensive parts that allow the use of manual processing. Electroless plating differs from other metal coating techniques (electroplating, vacuum metallizing, hot dip galvanizing) in that the substrate initiates the autocatalytic chemical reduction process. Some of the metals, such as nickel, are catalytic when immersed in an electroless plating solution, while steel and aluminum become catalytic by the formation of an immersion nickel deposit in the electroless nickel bath. Other metals, such as copper, are passive and require activating steps in order to initiate the electroless nickel plating process. It should be noted that even naturally active surfaces can become passive when contaminated by foreign residues or oxide layers. For this reason, surface preparation for electroless nickel plating requires the highest degree of care and control of all metal finishing procedures.