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Spring Coatings and Surface Treatments

spring coatings and surface treatment

Many coatings and surface treatments are available for metals. Some are primarily responsible for corrosion prevention while others are intended to improve surface hardness or wear resistance. Coatings are also used to change dimensions (slightly) and to alter some physical properties such as reflectance and color. The chart below shows various types of coatings and treatments for metals.

Electropolishing

Electropolishing is used for polishing of metal parts. The principle is just the reverse of that of electroplating. The workpiece is made the anode in an electrolyte, with a cathode added to complete the electrical circuit. In the resulting deplating, material is removed most rapidly from raised, rough spots, producing a very smooth, polished surface.  This process is primarily used for obtaining mirror-like surfaces from highly smooth initial surfaces. A final finish of less than 0.05 µm can be obtained if the initial surface roughness (root mean square) does not exceed 0.18 to 0.20 µm. Electropolishing is used for polishing stainless steel sheets and parts.

Tumbling

A tumbling (vibratory finishing) machine is an open-topped tub or bowl mounted on springs, usually lined with polyurethane. Vibratory action is created either by a vibratory motor attached to the bottom of the container, by a shaft or shafts with eccentric loads driven by a standard motor, or by a system of electromagnets. Tumbling can help to deburr metals, clean parts, or help to make a brighter spring finish.

Bowl vibrators are round bowl shaped and have a container mounted on springs. Vibratory action is imparted to the bowl by eccentric weights mounted on a vertical shaft at the center of the bowl. When activated, a spiral motion is imparted to the mass of parts and media. The major advantage of bowl vibrators is that an integral separation system can be incorporated. To separate parts from media, a dam is placed in the channel so that parts and media are forced up and over. At the top of the dam is a screen over which parts and media pass. Media fall through the screen back into the vibrator bowl. Parts are deflected off the screen into a collection hopper or conveyor.

Shot Peening

Shot peening is a method of cold working in which compressive stresses are induced in the exposed surface layers of metallic parts by a stream of shot, directed at the metal surface at high velocity under controlled conditions. It differs from blast cleaning in primary purpose and in the extent to which it is controlled to yield accurate and reproducible results. Although shot peening cleans the surface being peened, the major purpose of shot peening is to increase fatigue strength.

Media used for peening can be iron, steel, or glass shot, or cut steel or stainless steel wire. Metallic shot is designated by numbers according to size. Shot numbers, as standardized by MIL-S-13165, range from S70 to S780. The shot number is approximately the same as the nominal diameter of the individual pellets in ten thousandths of an inch. The effectiveness of the shotpeening operation is measured via the almen strip. This is a thin flat piece of steel that is clamped to a solid block and exposed to the blast of shot, which produces a curvature. The extent of this curvature on a standard sample serves as a means of measurement of the intensity of the peening.

Electroplating

Electroplating involves the creation of a galvanic cell in which the part to be plated is the cathode and the plating material is the anode. The two metals are placed in an electrolyte bath and a direct current applied from anode to cathode. Ions of the plating material are driven to the plating substrate through the electrolyte and cover the part with a thin coating of the plating material.

Steels, nickel and copper based alloys, as well as other metals are readily electroplated. Two approaches are possible. If a more noble (less active) metal is plated onto the substrate, it can reduce the tendency to oxidize as long as the plating remains intact to protect the substrate from the environment. Tin, nickel, and chromium are often used to electroplate steel for corrosion resistance. Chrome plating also offers an increase in surface hardness to HRC 70, which is above that obtainable from many hardened alloy steels. Unfortunately, any disruptions or pits in the plating can provide nodes for galvanic action if conductive media (such as rainwater) are present. Because the substrate is less noble than the plating, it becomes the sacrificial anode and rapidly corrodes. Electroplating with metals more noble than the substrate is seldom used for parts that will be immersed in water or other electrolytes.

Alternatively, a less noble metal can be plated onto the substrate to serve as a sacrificial anode which will corrode instead of the substrate. The most common example of this is zinc coating of steel, also called galvanizing. The zinc or cadmium coating will gradually corrode and protect the more noble steel substrate until the coating is used up, after which the steel will oxidize. Zinc coating can be applied by a process called "hot dipping" rather than by electroplating, which will result in a thicker and more protective coating recognizable by its "mother-of-pearl" appearance. A caution about electroplated coatings is that hydrogen embrittlement of the substrate can occur, causing significant loss of strength. Electroplated finishes should not be used on parts that are fatigue loaded. Experience has shown that electroplating severely reduces the fatigue strength of metals and can cause early failure.

Hydrogen Embrittlement - Whenever carbon steel is pickled in preparation for plating or during some electroplating processes, hydrogen can become absorbed into the material. While cracks can develop in the pickling or plating bath, more often they appear when the plated springs are in service. The hazard of hydrogen embrittlement becomes more acute when there is (1) high stress concentration, (2) high Rockwell hardness, or (3) high carbon content. Tempered materials are particularly susceptible. To relieve embrittlement, the springs must be baked immediately after plating to drive the hydrogen out of the material.

Electroless Plating

Electroless plating puts a coating of nickel on the substrate without any electric current needed. The substrate "cathode" in this case (there is no anode) acts as a catalyst to start a chemical reaction that causes nickel ions in the electrolyte solution to be reduced and deposited on the substrate. The nickel coating also acts as a catalyst and keeps the reaction going until the part is removed from the bath. Thus, relatively thick coatings can be developed. Coatings are typically between 0.001 in and 0.002 in thick. Unlike electroplating, the electroless nickel plate is completely uniform and will enter holes and crevices. The plating is dense and fairly hard at around 43 HRC. Other metals can also be electroless plated but nickel is most commonly used.

Chemical Coatings

The most common chemical treatments for metals range from a phosphoric acid wash on steel that provides limited and short-term oxida­tion resistance, to paints of various types designed to give more lasting corrosion pro­tection. Black oxide is a lower cost option to form a corrosion protective barrier over various types of steel, stainless steel or copper substrates. Black oxide can also dull surfaces where light reflection is undesirable.

 

Platings
Process Commercial Specifications Classes Available Finishes/Grades Available Chromate Conversion Colors Available Primary Purpose
Cadmium
Plating
QQ-P-416

AMS-QQ-P-416

Class I - 0.0005" minimum
thickness
Class II - 0.0003" minimum
thickness
Class III - 0.0002" minimum
thickness
Type I - As plated
Type II - With chromate treatment
Type III - With phosphate treatment
Colorless
Irridescent
Bronze
Brown
Olive Drab
Yellow
Forrest Green
Primarily used to protect steel and cast iron against corrosion.
Chrome
Plating
QQ-C-320

AMS-QQ-C-320

Class I - Corrosion protective plating
Class II - Engineering plating
Type I - Bright Finish
Type II - Satin Finish
- The metal so produced is extremely hard and corrosion resistant. The process is used for applications where excellent wear and/or corrosion resistance is required.
Copper
Plating

MIL-C-14550B

AMS 2418
Class 0 - 0.001" - 0.005"
Thickness
Class 1 - 0.001" minimum
thickness
Class 2 - 0.0005" minimum
thickness
Class 3 - 0.0002" minimum
thickness
Class 4 - 0.0001" minimum
thickness
- - Good corrosion resistance and conductivity.
Gold
Plating

MIL-G-45204C

Class 00 - 0.00002" minimum thickness
Class 0 - 0.00003" minimum thickness
Class 1 - 0.00005" minimum thickness
Class 2 - 0.0001" minimum thickness
Class 3 - 0.0002" minimum thickness
Class 4 - 0.0003" minimum thickness
Class 5 - 0.0005" minimum thickness
Class 6 - 0.0015" minimum thickness
Type I - 99.7% gold minimum
Type II - 99.0% gold minimum
Grade A - 90 Knoop maximum
Grade B - 91 - 129 Knoop
Grade C - 130 - 200 Knoop
Grade D - 201 Knoop and over
- Good corrosion resistance and high tarnish resistance. Solderability and conductivity are excellent.
Nickel
Plating
QQ-N-290

AMS-QQ-N-290

Class I - Corrosion protective plating
Class II - Engineering plating
Class I - Grade A through
G (0.0016" - 0.0002" Thickness)
- Used extensively for decorative, engineering, and electroforming purposes.
Silver
Plating

QQ-S-365D

ASTM B700
Grade A - Chromate
post-treatment
Grade B - No supplementary
treatment
Type I - Matte
finish
Type II - Semi-bright
finish
Type III - Bright
finish
- Good corrosion resistance and will tarnish easily. Solderability and conductivity are excellent.
Tin
Plating
ASTM B545

MIL-T-10727C

Type I - Electroplated
Type II - Hot dipped
- - Good corrosion resistance and excellent solderability.
Vacuum
Cadmium

MIL-C-8837B

AMS-C-8837
Class I - 0.0005" minimum
thickness
Class II - 0.0003" minimum
thickness
Class III - 0.0002" minimum
thickness
Type I - As plated
Type II - With chromate
treatment
Type III - With phosphate
treatment
Colorless
Irridescent
Bronze
Brown
Olive Drab
Yellow
Forrest Green
Primarily used to provide corrosion resistance to parts free from hydrogen contamination and possible embrittlement.
Zinc
Plating

ASTM B633

Service Condition 1 (Fe/Zn 5) -
mild conditions, 5mm
thickness
Service Condition 2 (Fe/Zn 8) -
moderate conditions, 8mm
thickness
Service Condition 3 (Fe/Zn 12) -
severe conditions, 12mm
thickness
Service Condition 4 (Fe/Zn 25)-
very severe conditions, 25mm
thickness
Type I - As plated
Type II - Colored chromate conversion coatings
Type III - Colorless chromate conversion coatings
Type IV - Phosphate conversion coating
Colorless
Blue
Olive Drab
Yellow
Good corrosion resistance.

 

Chemical Conversion Coatings
Process Commercial Specifications Classes Available Finishes/Grades Available Chromate Conversion Colors Available Primary Purpose
Black
Oxide

MIL-C-13924

Class 1 - Alkaline oxidizing process
Class 2 - Alkaline chromate oxidizing
Class 3 - Fused salt oxidizing process
Class 4 - Alkaline oxidizing process
Supplementary oil treatment per

MIL-C-16173

- A uniform, mostly decorative black coating. Limited corrosion resistance.
Phosphate
Coating
(Light)
TT-C-490E - Type I - Zinc phosphate spray application Zinc phosphate immersion or dip application
Type II - Aqueous iron phosphate
Type III - Organic pretreatment coating

(MIL-C-8514)

Type IV - Discontinued
Type V - Zinc phosphate
- Type I - All-purpose pretreatment prior to painting
Type II and IV - For parts to be formed after painting
Type III - Size and shape preclude Type I, II, IV
Type V - Zinc phosphate
Phosphate
Coating
(Heavy)

MIL-DTL-16232G

Class 1 (Type M/Z) -
Supplementary preservative treatment or coating
Class 2 (Type M) -
Supplementary treatment with lubricating oil
Class 2 (Type Z) -
Supplementary treatment with preservative
Class 3 (Type M/Z) -
No supplementary treatment
Class 4 (Type M/Z) -
Chemically converted (may be dyed to color as specified)
Type M - Manganese
Phosphate Base
Type Z - Zinc
Phosphate Base
- Coating for medium and low alloy steels. Intended as a base for supplemental coatings which provide the major portion of corrosion resistance.

 

Electroless Plating
Process Commercial Specifications Classes Available Finishes/Grades Available Chromate Conversion Colors Available Primary Purpose
Electroless
Nickel

MIL-C-26074F

AMS 2404C
AMS 2405B
Class 1 - As plated
Class 2 - Heat treated
Grade A - 0.001" minimum
thickness
Grade B - 0.0005" minimum
thickness
Grade C - 0.0015" minimum
thickness
- Used to deposit nickel without the use of an electric current.

 

Lubrication
Process Commercial Specifications Classes Available Finishes/Grades Available Chromate Conversion Colors Available Primary Purpose
Solid
Film
Lubrication

MIL-L-46010

- Color 1 - Natural product color
Color 2 - Black
- Used to reduce wear and prevent galling, corrosion, and seizure of metals.

 

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