Covering types of iron plating｜JIS Standard and Optimal Flow Evaluation Summary

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As a machine designer, it is not uncommon for you to have confidence in the design of functions and dimensions, but to have doubts about the selection of the final surface treatment.　　"Will this part really rust?" Is there any risk of hydrogen embrittlement? "How can I ensure optimal corrosion resistance while keeping costs down?" These questions are common concerns for many designers.

There are many different types of plating, and it is not easy to correctly understand their characteristics, comparisons, and the meaning of JIS symbols and reflect them in drawings.

Many websites tend to end up listing chemical reaction formulas and catalog specifications for plating, but this article is thoroughly focused on the "mechanical designer's viewpoint" and summarized with an awareness of "first of all, a broad and shallow viewpoint"! This article describes specific problems that occur onsite such as "out of dimension tolerance," "interference during assembly" and "delayed fracture. This section explains what should be considered at the design stage and how to describe them in the drawings in order to avoid such problems that may occur in the field.

From the flow of plating selection, to precautions in shape design, to the trade-off between cost and performance, this article covers practical knowledge based on JIS standards and actual processing conditions in Japan. We hope this article will help you select the most suitable plating and be able to deal with processors on an equal footing.

Basic knowledge of the types of plating applied to steel parts

Ionization tendency and corrosion mechanism of iron

Why do steel parts rust?Understanding the underlying causes is the starting point for proper corrosion protection design.　　Iron (Fe) is an energetically very active metal and exists in nature in a stable state called "iron oxide (iron ore).　　Although it is given a great deal of energy in the iron-making process to become "metallic iron," it is constantly emitting electrons and exerting thermodynamic forces to return to its original stable oxide state.The degree of this "tendency to emit electrons and become ions" is indicated by the ionization tendency.

When iron processed as a machine part comes into contact with oxygen and moisture (electrolyte solution) in the air, microscopic "local batteries" are formed on the surface.

1. Anodic reaction:A portion of the iron surface becomes an anode, and iron atoms lose their electrons (e-) and dissolve into the water film as iron ions (Fe2+). This is the beginning of corrosion.
2. Cathodic reaction:The emitted electrons move through the metal to the cathode, where they react with water and oxygen to form hydroxyl groups (OH-).
3. Rust formation:Dissolved iron ions and hydroxyl groups combine to form iron hydroxide, which eventually becomes red rust (e.g., ferric oxide Fe2O3).

The essence of the plating process is how to physically and chemically shut down this electrochemical circuit, which can be described as a "return-to-corrosion instinct.　　Designers must consider the presence of moisture and condensation in the operating environment and provide an optimal barrier to stop this ionization process.

Reference source: Sanwa Plating Industry Co.https://www.sanwa-p.co.jp/）

*The "Plating Q&A" and technical columns on the above sites and others provide detailed explanations of corrosion cases and the basics of plating.

Rust prevention by sacrificial anticorrosion action of zinc

Galvanizing is the most typical and cost-effective method of corrosion prevention for steel.　The main reason why this plating is chosen is that zinc is a metal with a greater ionization tendency (more base) than iron.　　When a zinc coating is formed on a ferrous material, even if the ferrous base material is exposed due to scratches during use, the surrounding zinc will ionize and dissolve preferentially over the iron.　　This phenomenon keeps electrons supplied to the iron and keeps the iron in an electrochemically reduced (cathodic) state, so corrosion does not progress.This is called "sacrificial anticorrosion action.

Thanks to this action, galvanization can strongly inhibit the formation of red rust even in the presence of some pinholes and abrasions.With paint or nickel plating, moisture that enters through scratches can spread rust internally and cause the paint film to swell, but such phenomena are unlikely to occur with galvanizing.　　This makes zinc plating a reliable choice for ferrous parts used outdoors or in humid environments, such as bolts, nuts, and other fastening parts, construction hardware, and automotive undercarriage parts.However, since zinc itself is very easily oxidized, the chromate treatment described below is essential to reduce its wear and tear.

Principle of coating anti-corrosion action of nickel, etc.

Plating such as nickel, chromium, and copper protect steel using a different approach than zinc.　　These metals either have a smaller ionization tendency (nobler) than iron or have the property of forming a dense passive film on the surface to stabilize it.　　The corrosion protection mechanism by covering the iron surface with these metals is called "coated corrosion protection action. This principle consists of completely shielding the iron from the environment (oxygen and moisture) by means of a physical barrier.

The greatest advantage of coated corrosion protection is that the coating itself is resistant to discoloration, and beautiful luster and high chemical resistance can be maintained for a long period of time.　　However, this method also carries a fatal risk.　　If there are minute holes (pinholes) or cracks in the coating and moisture penetrates through them, a strong corrosion cell is formed between the iron and the coating metal.　　In this case, the iron, which has a high ionization tendency, becomes the anode, and "pitting corrosion," in which corrosion progresses intensively in a narrow gap, occurs.Since it does not have sacrificial corrosion protection like zinc, once rusting starts, it progresses rapidly internally, causing plating to peel.　　Therefore, if nickel plating, etc. is employed for corrosion prevention purposes,Sufficient film thickness (e.g., generally 20 μm or more) to eliminate pinholesDesign considerations are essential, such as using a multilayer structure with copper plating on the substrate to improve adhesion and coverage.

RoHS Directive and Hexavalent Chromium Free Status

In the past, chromate treatment, which is mainly composed of hexavalent chromium, was widely used as a passivate treatment to dramatically increase the corrosion resistance of zinc coatings.　　Hexavalent chromium has an excellent "self-healing" property, chemically repairing scratches on the coating to provide long-term corrosion protection.　　However,Hexavalent chromium is carcinogenic to the human body and has an extremely high environmental impactIt is.　　This has led to their use being severely restricted or even prohibited by the European RoHS Directive (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment) and ELV Directive (End of Life Vehicle Directive).

Currently, Japanese industry has almost completed the conversion to the environmentally friendly "trivalent chromate.　　Trivalent chromate has a low environmental impact because it does not contain hexavalent chromium, and recent technological innovations have led to the development of treatment solutions with corrosion resistance equivalent or superior to that of conventional hexavalent chromium treatment.　　When designers simply describe "chromate" or "unichromate" in their drawings, there is a risk that some processors will use old stock of hexavalent chromium-treated products or that there will be discrepancies in recognition.　　Therefore,It is standard practice for modern machine designers to clearly indicate on the drawings that the part complies with environmental regulations by clearly stating "trivalent chromate (RoHS compliant)" or "trivalent white" or "hexavalent chromium free".The first two are the following.

Difference between white and red rust and salt spray test

As a quantitative indicator of the performance evaluation of plating, especially corrosion resistance, it is defined in JIS Z 2371 asNeutral salt spray testis commonly used.　　In this test, 5% salt water is continuously sprayed in a sealed tank, and the time required for rust to form on the specimen is measured.　　It is important to note that there are two types of rust generated, "white rust" and "red rust," and each has a different meaning.

White rust is a white powdery rust mainly composed of zinc hydroxide, etc., that occurs mainly due to oxidation of the galvanized layer itself.　　This is evidence that the zinc is performing its sacrificial corrosion prevention function soundly, but since it is detrimental to aesthetics, it is necessary to retard the occurrence of rust by chromate coating.　　On the other hand, red rust is iron oxide generated by corrosion of the bare iron, indicating that the corrosion protection capability of the plating has reached its limit and the iron has begun to be attacked.　　In design specifications, it is common to set standards for each stage, such as "72 hours for white rust to occur and 240 hours for red rust to occur.　　By understanding these values, you will be able to select the appropriate plating grade according to the severity of the operating environment.

Typical types of plating for ferrous metals and detailed properties

Electrogalvanization and trivalent chromate

As an anticorrosion treatment for ferrous parts, electro-galvanization offers the best balance between cost and performance.　　It is standardized by JIS H 8610, and grades are classified according to film thickness.Generally, "Class 2 (5 µm)" or "Class 3 (8 µm)" is selected for components of automatic and industrial machinery to provide sufficient corrosion resistance while avoiding excessive quality.　　Since zinc plating by itself is active and causes white rust immediately, it is always used in combination with "chromate treatment," which is a conversion treatment.

Current mainstreamtrivalent chromate　There are several variations in appearance and characteristics,Screws available for general purchase　They are used for such purposes as　　Designers should use these differently depending on the application.　　The following table summarizes typical types of trivalent chromates and their characteristics.

Table 1: Types and characteristics of trivalent chromate

Reference source: Sanwa Plating Industry Co.https://www.sanwa-p.co.jp/）
Reference source: Wakayama Corporation (Japanese only)https://www.wakayamapp.jp/）

Thus, even trivalent chromate varies in corrosion resistance and appearance depending on the type of treatment solution.　　In most cases, trivalent white is applied unless otherwise specified, but if higher corrosion resistance is required or a black design is needed, this should be clearly indicated on the drawings.

Electroless nickel plating and Kanizen plating

Electroless nickel-phosphorus plating, as defined in JIS H 8645, is a method of depositing a nickel film using a chemical reduction reaction without the use of electrical energy.　　Japan Kanizen Co.In the field, it is sometimes referred to by the common name "Kanizen plating" due to the history of the spread of this technology in Japan by the "Kanizen Plating Co.　　The most important feature of this plating is that it is not affected by current distribution as electroplating is,This means that the film thickness is extremely uniform, even for complex shapes.　　Tolerance tight holes and the inner surfaces of intricate channels,Even plating can be applied to areas that cannot be handled by electroplating.

In addition, phosphorus (P) is contained in the film, and heat treatment dramatically improves hardness.　　The table below shows the basic properties of electroless nickel plating and the change in hardness with heat treatment.

Table 2: Basic Properties and Heat Treatment Effects of Electroless Nickel Plating (Medium Phosphorus Type)

Reference source: Japan Kanizen Corporation (https://www.kanigen.co.jp/）
Reference source: JIS H 8645 Browse (https://www.jisc.go.jp/）

Designers can achieve highly functional parts by selecting electroless nickel for parts that require precision and noting "with heat treatment" on the drawing if further wear resistance is required.　　However, care must also be taken in the selection of base metal material, as the base metal may be deformed or tempered depending on the heat treatment temperature.

Hard chrome plating with excellent wear resistance

Industrial chrome plating, commonly known as "hard chrome plating," is an extremely hard plating with a low coefficient of friction as defined by JIS H 8615.　　Its hardness reaches 800-1000 HV in Vickers hardness, much harder than hardened steel, so rods, molds, and rolls of hydraulic cylinders exposed to intense wear environments,Linear shaftIt is widely used in such applications asFilm thickness can be controlled over a wide range, from flash plating of a few microns to thicker plating of several hundred microns for repairing worn parts.

However, hard chrome plating has a weakness in that it has extremely poor "stickiness.　　Due to the characteristics of electroplating, the current is excessively concentrated on the sharp edges and protrusions of the product, causing the plating to rise abnormally in those areas.　　On the other hand, plating is hardly deposited in recesses or deep inside the bore.　　Therefore,When precise dimensional accuracy is required, the design is generally based on the assumption that after plating, a process of cylindrical polishing or buffing is performed to remove excess raised portions and adjust the dimensions.　　The drawing should include not only the finished dimensions after plating, but also instructions that take the process into consideration, such as "plating thickness of 00μm must be secured" or "polished finish.

Treatment with black dye (iron tetroxide film)

Black oxide is not strictly speaking plating (metal deposition), but a conversion treatment in which the surface of iron is changed to black iron oxide (Fe3O4: iron tetroxide/magnetite) by a chemical reaction.　　It is known as a treatment equivalent to JIS H 8622.　　The treatment is performed in a high-temperature alkaline solution to form a very thin oxide film of about 1 to 2 μm on the surface. The greatest advantage of this treatment is that it hardly changes the dimensions of the material.

On shafts with precise fit tolerances, such as H7 and g6, and in areas where you want to maintain the accuracy of screw fitting,Black oxide is the best option when the increase in film thickness due to plating cannot be tolerated.　The black surface also prevents diffuse reflection of light and is therefore preferred for internal parts of optical equipment and stands for measuring instruments.　　On the other hand,The rust-preventive ability is very low, and the film by itself rusts quickly due to moisture in the air.Therefore, it is necessary to always apply and impregnate rust-preventive oil after treatment,Assumed to be used in an environment that does not lose oil film during use (inside hydraulic equipment, jigs that are constantly maintained, etc.)will be.

Sliding property improvement by manganese phosphate treatment

The manganese phosphate treatment is called "liebrightIt is a type of chemical conversion treatment also known by the trade name "phosphate coating" and is related to JIS H 8617 (phosphate coating).　　It produces a crystalline film of manganese phosphate on the steel surface, which is thicker (about 5 to 15 μm) than black oxide.　　This film is an aggregate of fine crystals and has excellent "oil retention" properties that allow it to retain a large amount of lubricating oil in the gaps between the crystals.

Because of this property, the material can be used in sliding parts such as gears, pistons, cams, and bearings that move by metal-to-metal contact,Widely used to improve initial blending and prevent seizure (galling).　　Since the coating itself is softer than metal, it wears moderately during blending operation immediately after start of use, creating a smooth sliding surface that fits the mating material.　　Therefore, when using it on a part with tight tolerances, it is necessary to set the material dimensions to the minus side in consideration of the film thickness, or to factor in dimensional changes after blending.

Performance and Risk Management by Plating Type

Threadability and Properties of Electroplating

When using electroplating, one of the most important characteristics that designers need to be aware of is the "adhesion property" (uniform electrodeposition).　In electroplating, electricity is applied to the product (cathode) in the plating solution to attract and deposit metal ions.　　Due to the characteristics of electricity, the current tends to concentrate on the "shortest distance" or "sharpest point" with the least resistance.　　Conversely, the current does not flow around concave areas, inside of holes, and shadow areas, resulting in thin or no plating at all.

This variation in film thickness is the primary cause of deviation from the designer's intended dimensional tolerances.　　For example, even if the film thickness is controlled at the center of a flat plate, it may be more than twice as thick at the edges, causing interference during assembly.　　To prevent this, it is effective to add large rounded corners at the design stage to mitigate current concentration, or to consult with the plating vendor about the use of "auxiliary anodes" (additional electrodes installed in locations where the current is difficult to reach).　　However, since auxiliary anodes lead to higher costs, reviewing the shape itself is the most cost-effective solution.

Film thickness uniformity of electroless plating

Electroless plating solves the weakness of electroplating, which is the non-uniformity of film thickness.　　In this method, the film growth is not caused by electrical attraction, but by a catalytic reaction of chemicals (reducing agents) in the plating solution on the metal surface.　　Therefore, as long as the liquids are in contact and the temperature and concentration are properly controlled,Plating deposits at an equal rate on all surfaces of the product.

This "smoothness" allows for uniform film thickness even in intricate internal flow paths of manifolds, precision threads, and tolerance hole bores.　　Designers can specify electroless nickel plating for parts with tight geometric tolerances or where a uniform corrosion protection layer is required,The risk of dimensional problems and quality defects during manufacturing can be greatly reduced.　　However, in holes with cul-de-sacs, there is a possibility that the reaction will not proceed (depletion of the nickel component in the liquid) due to retention of the liquid, so it is still necessary to consider through-holes or drain holes to allow the liquid to circulate.

Vickers hardness (HV) indicating surface hardness

Surface hardness is an indispensable index for evaluating the durability of mechanical parts, especially wear and scratch resistance.　　Plating hardness is generally expressed in terms of Vickers hardness (HV).　　Designers should select the appropriate plating hardness depending on the load and contact conditions to which the part is subjected. The table below shows a hardness comparison of the main plating and materials.

Table 3: Comparison of Vickers hardness (HV) of main plating and materials

Reference source: JIS B 7725 (Vickers hardness test) compliant data
Reference source: Japan Kanizen Corporation (https://www.kanigen.co.jp/）

As can be seen from the table, electrolytic zinc plating is softer than the material steel, making it unsuitable for sliding parts.　　On the other hand, hard chrome and electroless nickel after heat treatment have very high hardness.　　It is important for designers to understand this difference in hardness as well as cost, and to choose the surface treatment that best suits the application (whether it is simply rust prevention or must withstand severe wear).

Danger of delayed fracture due to hydrogen embrittlement

The most alarming and unseen fear of mechanical designers is the "Hydrogen embrittlement".　　This occurs when atomic hydrogen generated during the plating process (during pickling and electroplating) enters and is absorbed into the steel. Over time, the hydrogen that has entered the steel collects in areas where stress is concentrated (such as the bottom of a bolt trough or the flexure of a spring), significantly reducing the metal's bonding strength and increasing the pressure by gasifying inside, causing cracks to occur.

The frightening aspect of this phenomenon is that abnormalities cannot be detected by visual inspection or tensile testing immediately after plating.　　After the product has been assembled and a constant load has been applied for several hours, days, or even months, it will suddenly rupture with a "snap" without warning.　　This is called "delayed fracture.　　In particular, high-strength steels (bolts with tensile strength exceeding 1000 MPa, tempered SCM materials, spring steels, etc.) are extremely susceptible to hydrogen embrittlement,Easy adoption of electroplating risks serious accidents.

Brittleness removal by baking process

The only and greatest measure to avoid the risk of delayed fracture due to hydrogen embrittlement is the "Baking processThis is the "dehydrogenation" process.　　This is a process in which parts are placed in a furnace immediately after plating and held at high temperatures for a certain period of time to release hydrogen that has penetrated inside (dehydrogenation) to the outside.　This process makes it possible to safely use even high-strength parts.

Table 4: General Baking Process Conditions and Recommendations

Reference source: Northeast Giken Kogyo Co.https://hokutohgiken.co.jp/）
Reference source: JIS B 1051 (Mechanical properties of carbon and alloy steel fastening parts)

In drawings that use high-strength materials, designers are responsible not only for specifying the type of plating, but also for specifying in the notes column that "baking treatment (at least 4 hours at 200°C) shall be performed immediately after plating. The presence or absence of this line can make a world of difference in product safety.　　In addition, baking may cause a slight decrease in corrosion resistance due to the loss of moisture in the chromate film, so this trade-off must also be taken into consideration.

Design precautions and measures for each type of plating

Measures against edge build-up at corners

Thickening of the film at the corners, an unavoidable physical phenomenon in electroplating, is a common cause of assembly problems.　　Even if plated aiming for a film thickness of 10 μm on flat areas, it may grow to 30 μm or even 50 μm or more on the edges.　　This causes problems such as precision right-angle blocks that cannot be inserted into gauges or chamfered parts of shaft parts that are too fat to be inserted into holes.　　Abnormally grown plating is also brittle, causing "chipping" upon impact during assembly, which in turn causes secondary damage to the sliding parts as hard foreign particles.

As a design measure, it is recommended that the corners of parts be "R-surfaced" (rounded) whenever possible, rather than "pin-cornered" or "C-surfaced" (chamfered at 45 degrees).　　By adding R, the current concentration point is dispersed and abnormal film thickness growth can be suppressed to some extent.　　Ideally, it should have a rounded edge of R0.5 or more, or at least R0.3.　　For blades, reference surfaces, etc., where sharp edges are absolutely necessary, the shape should either be polished after plating or the surface should be shaped,Consider changing to electroless nickel plating, which does not cause film thickness enlargement in the corners.

Plating design considering dimensional tolerances

Plating Thickness" has a direct impact on the dimensional tolerances of design drawings.　　It is important to note that the plating thickness is an increase in the "radial" direction, so the diameter (shaft or hole) is affected twice as much.　　For example, a 10 µm thick plating will increase the diameter of a shaft by 20 µm and decrease the diameter of a hole by 20 µm.　　This would not be a problem for loose parts with a tolerance of ±0.1mm, but for tolerances controlled in the 10-20μm range, such as h7 and H7, the plating thickness alone is highly likely to result in an out-of-tolerance (NG).

To prevent this, dimensional design that accounts for "plating allowance (shiro)" is necessary.　　Specifically, the dimensions before machining are adjusted in advance so that the finished dimensions after plating will be in the center of the drawing tolerance. For shafts, we aim for "finished diameter - plating thickness x 2", and for holes, we aim for "finished diameter + plating thickness x 2".It is possible to prevent problems by clearly stating on the drawing that "dimensional instructions are to be given after plating" or by writing "dimensions before plating: Φ0" as an instruction to the processor.　　It is also important to remember that the plating thickness itself can vary by ± a few micrometers, so it is important to calculate the buildup of tolerances.

Liquid drainage hole placement and design criteria

When plating pipe frames, box-shaped cans, blocks with bag holes, etc., to prevent plating solution or pretreatment solution from remaining insideThe design of the "drain hole" is critical to both quality and safety.　　If there are enclosed spaces or cul-de-sacs, liquid can be carried out when moving between processes, contaminating the next tank, or acid left inside the product can later seep out and cause severe rusting.　　Even more dangerous, if the product is placed in a baking or drying oven with moisture remaining inside, rapid steam expansion can lead to an accident in which the product explodes.

The following principles apply to the design of liquid drainage holes

1. Diagonal arrangement: 1 Drill at least two holes, one at the top and one at the bottom (diagonal) of the product when it is hung, to provide an entrance and exit for the liquid.
2. Size: 1.5 Considering the surface tension and viscosity of the liquid, the hole diameter should be at least φ6 mm, and if possible, φ10 mm or larger. Small holes will not allow air to escape and liquid to enter.
3. Position: (1) Placement in a position that does not allow for structural "air pockets" where air can accumulate.

Table 5: Recommended liquid drainage holes

Reference source: Takita Corporation (Japanese only)https://takita-dnk.co.jp/）
Reference source: Japan Hot Dip Galvanizing Association (structural guidelines)

Even if holes are not desired for design reasons, it is necessary to look for invisible surfaces to place holes or to consider corrosion prevention methods other than plating (e.g., painting).A naive belief that "the site will take care of itself" can be the source of serious accidents.

Relationship between masking instructions and cost

When drawings indicate "no plating of threads" or "no plating of tolerance holes", "masking work" is required to protect these areas at the plating site.　　This process is mostly an analog process by hand, such as applying heat-resistant tape, packing rubber plugs, and inserting bolts.　　Therefore, depending on the shape of the part and the number of masking points, the labor cost for masking is often higher than the unit price for the plating process itself.

Design techniques to control costs include the following

• Utilization of post-processing:. Re-threading (exposing) the tap after plating is often cheaper than masking. However, since the rust-preventive power of the threaded part will be reduced, care such as applying rust-preventive oil after assembly is necessary.
• Relaxation of instructions:. Avoid overly strict instructions such as "Masking at exactly 0 mm from the edge of the hole," and instead set a tolerance range that is easy to work within, such as "No plating inside the hole and around the seat surface, but the boundary is up to the customer's discretion.
• Examination of special jigs:. For mass-produced products, instead of applying tape, a special masking jig that can be removed and attached with a single touch can be produced, which will reduce costs in the long run.

Cost vs. effectiveness (trade-off) perspective

The biggest concern in plating selection is the balance between performance and cost, or the trade-off decision.If all parts were plated with the finest electroless nickel or hard chrome plating, performance problems would not occur.　　However, this will drive up product costs and cause a loss of market competitiveness.　　On the other hand, if costs are cut too low and inappropriate electrogalvanization is used, the result will be corrosion claims in the market and rework costs due to assembly defects, resulting in huge losses.

Designers need to comprehensively analyze the importance of the components, the environment in which they will be used, their expected lifetime, and the frequency of maintenance to find the best compromise.

• Invisible internal components:Inexpensive electrogalvanization (grade 3) is sufficient.
• Precise positioning pins:Electroless nickel plating, even if expensive, is used to avoid problems.
• Cover for outdoor use:Even if the initial cost is high, choose highly corrosion-resistant plating (such as zinc-nickel alloys) or cationic electrodeposition coating to make the product maintenance-free.

Thus, the discerning ability to identify the "right person for the right job" is the skill required of a professional mechanical designer.

Flow for selecting the best type of plating

Summary: Procedure for selecting the best type of plating

Integrating the knowledge explained so far, a practical (basic) flow for selecting the best plating for steel parts is shown below.　　By thinking along this procedure when drawing, you will be able to select the best surface treatment without hesitation.

1. What is the purpose of plating?
   • Rust prevention only:Go to Step 2.
   • Function (accuracy, hardness, sliding) is required:Go to Step 3.
   • Dimensionally Maintained (zero film thickness):. Consider black dyeing (but anti-rust oil management is essential).
2. [Determination of environment and corrosion resistance level] Where to use?
   • Indoor and general environment:Electrogalvanized (Ep-Fe/Zn 8, trivalent white). This is the basic standard.
   • Humidity/light outdoors:Electrogalvanized film thickness increase (grade 3 to 4) or zinc-nickel alloy plating.
   • Harsh corrosive environment:Consider hot dip galvanizing (dobbing) or changing to stainless steel materials.
3. Narrowing down the functional requirements] What characteristics are needed?
   • Dimensional accuracy (tolerance holes, complex shapes):Electroless nickel plating (ELp-Fe/Ni-P).
   • Wear-resistant and high hardness:Hard chrome plating (ICr) or electroless nickel + heat treatment.
   • Sliding and initial familiarization:Manganese phosphate treatment (Leubright).
4. Can you protect safety and quality?
   • High strength material (1000 MPa or higher)? :If Yes, add "baking process" to notes.
   • Bag shape or welded construction? :If Yes, add "liquid drainage holes" to the design.
   • Tolerances are tight? :If Yes, adjust dimensional tolerances to account for "plating allowance".
5. Finalization and drawing instructions
   • Describe specifications accurately using JIS symbols.
   • Example: "Ep-Fe/Zn 8 (trivalent white)" "ELp-Fe/Ni(90)-P 10 [with heat treatment]"
   • If anything is unclear, do not make decisions on your own, but consult with the processor.

By repeatedly practicing this flow, the speed of decision making as a designer and the reliability of quality will be dramatically improved.　　Plating is an important process that marks the end of machine design.　　We hope that with correct knowledge and logical selection, you will create a machine that will be loved for a long time.

summary

• Corrosion of iron is an electrochemical battery action and plating is done to interrupt this circuit
• Zinc plating has a self-healing function that protects steel from scratches through "sacrificial corrosion protection".
• Nickel and chromium are "coated corrosion inhibitors" and provide a strong barrier as long as there are no pinholes
• Since hexavalent chromium is subject to regulation, the drawing clearly specifies "trivalent chromate (RoHS compliant)".
• Electrogalvanization is a standard corrosion protection treatment for indoor equipment with a good balance between cost and performance
• Electroless nickel plating has a uniform film thickness and is ideal for tolerance holes and parts with complex geometries
• Hard chrome plating has overwhelming hardness, but care must be taken to avoid current concentration at the edges (build-up)
• Black dye has little dimensional change but low rust-preventive power, while manganese phosphate is effective in improving the blending of sliding parts.
• Considering the poor "wraparound property" of electroplating, the corners are rounded to prevent abnormal film thickness.
• Plating on high-strength steels carries the risk of delayed fracture due to hydrogen embrittlement, so baking treatment is essential
• For parts with tight dimensional tolerances, adjust the material dimensions to allow for a change of twice the plating thickness (diameter).
• Bag-shaped parts are provided with diagonal liquid drainage holes to avoid residual liquid and the risk of steam explosion
• Masking instructions are the main cause of cost increases, so optimize by post-processing and relaxing tolerances
• Plating selection is determined based on a comprehensive evaluation of trade-offs between corrosion protection, function, cost, and risk.
• Understanding the JIS symbols correctly and communicating accurate specifications to processors is the key to preventing problems.

That's it.