Zinc Die Castings: Preventing White Etch Pitting

What is white etch pitting in zinc die castings

White etch pitting is a visible surface defect that appears as small, white, pitted areas on the face of a zinc die casting. It is a form of localized corrosion. It is not a casting defect that always forms under the die. It usually forms after the part leaves the die and enters the environment.

The defect is common in zinc die castings because the alloy is relatively reactive. It is more sensitive to corrosion than many other die cast alloys. The white color comes from zinc oxide and other corrosion products. The pitting happens where the metal surface breaks down at a microscopic level.

Engineers often notice the defect after the part has been in storage or in a humid environment. The pitting can appear on flat surfaces, corners, or areas where cleaning chemicals may have been left on the metal. The defect is usually not structural. It is a surface quality issue. However, it can fail cosmetic inspection. It can also be a sign of a larger process problem.

Common symptoms of the defect

White etch pitting usually shows up in a few specific ways. The first symptom is small white dots on the surface of the part. The second is a rough, pitted texture in those white areas. The third is a visible contrast between the white pitted area and the surrounding metal.

The defect can appear on one side of the part or on both. It can appear on a single part or across a whole batch. It can appear immediately after the part is cleaned or weeks later. The size of the pitting varies. Some spots are barely visible with the naked eye. Others are large enough to be seen without magnification.

In many cases, the defect is found during final inspection. The part is visually inspected. The inspector sees the white spots. The part is marked as a surface defect. The question then becomes how to prevent it from happening again.

Table of symptoms, causes and fixes

The table below lists common symptoms, likely causes, and corrective actions.

Symptom	Likely cause	What to do
Small white dots on flat surfaces	High humidity or salt exposure in storage	Control storage conditions and use protective coatings
White pitting on corners and edges	Residual cleaning chemicals left on the part	Improve cleaning and rinsing procedures
Pitting across a whole batch	Poor alloy chemistry or melt contamination	Test melt chemistry and review alloy charging
Pitting on one side of the part	Die surface contamination or coolant residue	Clean the die and review the die lubricant
Rough pitted areas after plating	Underlying corrosion or poor surface preparation	Inspect the pre-plating surface and control the environment

Likely causes of white etch pitting

The most common cause of white etch pitting is environmental corrosion. Zinc is a reactive metal. It reacts with oxygen, moisture, and certain chemicals. When these elements are present, the surface of the metal can corrode. The corrosion products can appear white.

Alloy chemistry is another major cause. Zinc die cast alloys contain small amounts of magnesium, copper, and iron. If the ratios are outside the accepted range, the corrosion behavior can change. A melt with too much magnesium may be more susceptible to corrosion. A melt with impurities from recycled scrap may contain contaminants that promote pitting.

Die maintenance is a third cause. The die surface can hold residual metal, coolant, or cleaning agents. These residues can react with the fresh metal during casting. They can also remain on the part after it is ejected from the die. When the part is exposed to moisture, these residues can cause localized corrosion.

Storage is a fourth cause. Parts are often stored in boxes, on racks, or in trays. If the environment is humid, the parts can corrode before they ship. If the parts are stored in salt-affected areas, the corrosion can happen faster. If the parts are in direct contact with other metals, galvanic corrosion can occur.

Prevention strategies

Preventing white etch pitting requires control at every stage of the process. The first stage is alloy control. The melt chemistry should be tested regularly. The ratios of zinc, magnesium, copper, and iron should be within the accepted range for the specific alloy. If the melt is out of spec, it should be corrected before casting continues.

The second stage is die maintenance. The die should be cleaned and inspected regularly. The die face should be free of residual metal and contamination. The die lubricant should be the correct type and concentration. The coolant should be clean and free of degradation products.

The third stage is post-casting cleaning. Parts should be cleaned to remove die lubricant, coolant, and other residues. The cleaning process should be controlled. Rinsing should be thorough. The parts should be dried properly. Residual water or cleaning chemicals should not be left on the surface.

The fourth stage is storage and handling. Parts should be stored in a controlled environment. Humidity should be managed. Salt exposure should be avoided. Parts should be protected from direct contact with other metals. Packaging should be chosen to reduce the risk of corrosion.

Inspection and testing

Inspection is the final step in preventing white etch pitting. Parts should be inspected after casting, after cleaning, and after storage. Visual inspection can catch many defects. Magnification can help find smaller pitting. Surface testing can check for corrosion products.

The inspection process should be documented. The date, time, inspector, and findings should be recorded. This helps trace the defect back to a specific batch or process step. If a defect is found, the process should be reviewed. The root cause should be identified. Corrective actions should be taken.

Testing can include chemical analysis of the melt. It can include surface testing for corrosion products. It can include environmental monitoring for humidity and temperature. These tests provide data that supports process control. They help prevent defects before they appear.

Common mistakes to avoid

One common mistake is ignoring the storage conditions. Parts may be cast and cleaned properly. They may be in good condition when they leave the die. But they can corrode in storage. Humidity and salt exposure can cause pitting after the fact.

Another mistake is not testing the melt chemistry. The alloy may be out of spec without anyone knowing. The melt may contain impurities that promote corrosion. Regular testing is needed to catch these problems.

A third mistake is poor die maintenance. The die may hold contamination that causes localized corrosion. The die lubricant may be the wrong type. The coolant may be degraded. These issues can cause defects that are hard to trace.

A fourth mistake is inadequate cleaning. Residual chemicals can remain on the part. They can react with moisture and cause pitting. The cleaning process should be controlled and documented.

Final check

White etch pitting is a preventable defect. It is caused by a combination of environmental, alloy, and process factors. The defect can be caught early with good inspection. It can be prevented with good process control. The key is to understand the cause and apply the right fix.

If you see white etch pitting, do not just reject the part. Find the cause. Check the melt chemistry. Check the die condition. Check the cleaning process. Check the storage environment. The defect is a signal. It tells you something is out of control. Fix the control. The defect will go away.