Technical description of salt spray testing for battery shrapnel

This article comes from the frontline experience of the Good Craftsman's shrapnel craft team. We have been dealing with the incoming materials and failure analysis of battery shrapnel for a long time. The conclusions in the following text are mostly based on the lessons learned from returned samples and customers' production lines.  
Many purchases neglect salt spray testing until the product rusts and has poor contact in the southern region of Huanan Tian, and the after-sales phone is called out, only then do they remember the process that was "saved" in the past.  
Salt spray testing is not a bonus point, but a "physical examination report" on how long shrapnel can survive in humid environments. Today, we will explain this matter clearly from the perspective of craftsmanship.

1. Corrosion, starting from a crack in the coating

Battery pellets are typically composed of phosphor bronze/spring steel/stainless steel substrates with electroplated coatings (nickel, gold, or tin). We often receive a type of failed sample in our laboratory: the substrate has developed red rust, the contact resistance has skyrocketed from the normal milliohm level to the ohm level, and the equipment is constantly on and off. The mechanism is not mysterious - in humid and salty air, the micropores and microcracks on the coating will become the breakthrough point for corrosion. The corrosive medium corrodes through the microporous coating, causing the substrate to rust and form an insulating oxide layer, resulting in a surge in contact resistance. Mild cases may result in occasional equipment malfunctions, while severe cases may involve bulk returns. Without salt spray verification, you never know if this batch of shrapnel can withstand a rainy season. This is not a matter of whether the material is good or not, but whether you have data to prove whether it is good enough.  

2. How do good craftsmen do salt spray testing

We don't treat salt spray as a formality. According to neutral salt spray (NSS) and ASTM B117; According to the national/international standard ISO 9227, the corrosion resistance of the shrapnel is verified, and the testing duration is determined based on the end use environment - the requirements are completely different between dry indoor and humid vehicle mounted environments. (Note: Salt spray is also divided into NSS neutral, AASS acetate spray, and CASS copper accelerated acetate spray. The latter two will be used in harsh environments.). ）More importantly, the system: relying on the German T Ü V Rheinland ISO9001 quality management system, skilled craftsmen are equipped with their own salt spray test chambers, which are controlled layer by layer according to more than 30 internal testing processes. Each shipment is accompanied by a traceable batch salt spray report. Salt spray is one of the hard levels, it does not meet the standards and will not leave the factory. In other words, salt spray is not just a 'test and see', but a hurdle that is written into the process and stuck before shipment.  

3. Salt spray level, corresponding to your usage environment

The stricter the usage environment, the higher the requirements for coating and testing: dry indoor environment (such as ordinary remote controls, electronic toys): relatively basic requirements; Wet/salt spray environments such as bathrooms, balconies, and cars: stricter measures should be taken, and gold or thick nickel plating should be preferred. The practice of a good craftsman is to first clarify your end use scenario, and then deduce which level of control and coating scheme to use. We do not advocate 'the longer the test, the safer it is' - time must match the real environment in order to be meaningful, graded according to needs, reliable and cost-effective.  

4. What to judge: Red rust and blistering

After salt spray, it mainly depends on two things: whether the substrate has red rust and whether the coating has bubbles - rather than simply who "bubbles" for longer. Within the same 48 hours, some people develop red rust while others remain intact, with the difference being in the quality of the coating and the compatibility of the solution. Red rust indicates that the substrate has corroded and is considered to be non compliant in terms of hardness; Bubbling means that the adhesion of the coating has failed, which is a precursor to separation. Either of the two usually does not pass the judgment. Good craftsmen use the terminal environment to deduce the testing duration and coating scheme, avoiding "testing for the sake of testing".  

5. The boundary of salt spray, don't be fooled by one qualification

Salt spray verification is about "resistance to corrosion", but it cannot verify "elasticity attenuation", nor does it mean true life prediction - laboratory accelerated corrosion is only the lower limit reference of corrosion resistance, and cannot cover all failure modes such as mechanical wear and sweat corrosion. There are two core life indicators for shrapnel: corrosion resistance and elastic life, which must be measured separately and cannot be replaced by each other. Good craftsmen list salt spray, elasticity, and coating thickness as parallel factory items, and will not ship any items that do not meet the standards - they will never use one item to cover up the hidden dangers of another.  

✅  Selection/Avoiding Pit List

Dry indoor use → Basic salt spray verification is sufficient
Moist/bathroom/car use → Strict verification is required
Harsh environment → Preferred gold or thick nickel plating
Before placing an order, please request the manufacturer to provide a factory salt spray report (including standards, duration, judgment photos, batch number). III. Frequently Asked Questions

Q1: How many hours does the salt spray test usually take?

There is no unified answer, it depends on the environmental level. Basic level (dry room) common 24-48 hour NSS; Tighten the time limit (for car/bathroom) to 96-240 hours, and some OEM specifications refer to ISO 9227, which can reach 480 hours. The key is to reverse the duration according to the terminal environment, rather than blindly pursuing numbers.  

Q2: Which is more severe, red rust or blistering?

Red rust is more severe - it indicates that the substrate (steel/iron) has corroded, posing a direct threat to conductivity and structural safety, and is considered a hard and unqualified product. Bubbling is a precursor to the separation of the coating and substrate, indicating issues with adhesion or coating integrity. If either of the two appears, it usually does not pass.  

Q3: Is gold plating necessarily better than nickel plating?

Gold plating has better conductivity and corrosion resistance (gold has high chemical stability and low contact resistance), but the cost is high; Thick nickel (such as Ni 5-10 μ m+) has a high cost-effectiveness in most humid environments and is also reliable with proper pre-treatment. Which one to choose depends on the budget and the strictness of the environment, not the more expensive the better.  

Q4: After passing the salt spray test, will it never rust?

can't. Salt spray is a laboratory validation of accelerated corrosion, representing the lower limit of corrosion resistance, but not equal to the actual life prediction; And it does not cover all failure modes (mechanical wear, sweat corrosion, elastic fatigue). The deliverables should be accompanied by parallel factory items such as elasticity and coating thickness.

Q5: How to determine if the supplier has really conducted salt spray testing?

Require factory salt spray report (including standard, duration, judgment photos, batch number); Check if it has third-party system certification (such as T Ü V ISO9001) endorsement; If necessary, request samples for self retesting or entrust third-party organizations (such as SGS, Radio and Television Metrology, etc.) to verify.  

Q6: Is there a relationship between the attenuation of shrapnel elasticity and salt spray?

Salt spray mainly measures corrosion resistance and does not directly measure elasticity. Elastic attenuation comes from material yielding, repeated compression, and stress relaxation, and requires separate testing for lifespan/insertion or compression permanent deformation. Two are independent and cannot be replaced by each other.
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