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Electric corrosion mechanism & Design for countermeasures

Detailed mechanism of electric corrosion and countermeasures against it/Design considering electric corrosion

 

In the previous column, we explained our policy on electric corrosion.

In this column, we describe the detailed mechanism of electric corrosion and the points that should be noted when designing.

The contents described here are excerpts from the reports and lecture materials of the Japan Electronics and Information Technology Industries Association (JEITA).

 

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Mechanism of electric corrosion

 

Electric corrosion is generally considered to be an electrochemical reaction that proceeds by the following mechanism.

(1) Moisture penetrates the protective coating of the resistor and reaches the resistive film under a high temperature/humidity atmosphere or due to dew condensation.

(2) When electricity is applied while moisture(water) is adhered to the resistive film, the water is electrolyzed.

(3) At this time, if any ionic impurities exist, electrolysis of water is accelerated.

(4) Hydroxide (-) ions generated by electrolysis of water move to the anode (+) and chemically react with the resistive film.

(5) The components of the resistive film change in quality and the physical properties change.

(6) Conductivity changes due to the changes in physical properties, causing an increase in resistance.

 

 

The above mechanism is expressed by a chemical formula as follows:

・For carbon film resistors

H2O(l) → H+ + OH–
C + 2OH– → CO2(g) + H2(g) +2e–

 

・For metal film resistors (nichrome type)

H2O(l) → H+ + OH–
Ni + 2OH– → Ni(OH)2 + NiO + H2O
２Cr + 6OH– → Cr2(OH)6 → Cr2O3 → ３H2O

 

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Q&A about measures against electric corrosion

Q1. Is it better to lower the resistance load factor?

A1. A lower resistance load factor is not always better. 
      Since electric corrosion is an electrochemical reaction, it is theoretically assumed
      that the larger the amount of current (the quantity of electrons),
      the faster the reaction will proceed. 
      However, when we conducted a reproduction experiment with a pressure cooker,
      as a result of comparing the load factors of 1% and 50% at the same temperature
     and humidity, the latter had a smaller resistance value change rate.
     We believe that this is because of evaporation and divergence of water due to the load heat.

 

Q2. The entire circuit board is encapsulated with resin to prevent infiltration of moisture.
      Is it effective?

A2. A certain effect can be expected, but care must be taken when handling the resin
     and selecting the resistance.  When encapsulating with resin, if any moisture attached to
     the circuit board and the parts on it is not sufficiently dried, the moisture may remain
     there and cause electric corrosion.  In addition, the flame-retardant components
     contained in the resin and the resin that changed due to hydrolysis may have some
     effect.  We recommend that you use metal glazed thick film resistors for circuit boards
     that require encapsulation with resin instead of using carbon resistors with high resistance
     (generally 100 kΩ or more).

 

Q3. What kind of resistor is recommended as a measure against electric corrosion?

A3. We have carbon resistors (HDM series) and metal film resistors (RNV series) with
     improved resistance to electric corrosion.  If higher reliability is required,
     we recommend products that use a metal glazed thick film for the resistive film.
    These products have the resistor structure that does not generate electric corrosion
    in principle, and their reliability against disconnection is extremely high.

 

・Metal glazed thick film products
       Anti Electrical Corrosion Resistors  AEC series
       Special Power Type Anti Surge Resistors  ASR/ASRM series
       Thick Film Chip Resistors  CR series
       Anti-Sulfurated Thick Film Chip Resistors  CRA series

 

 

 

Design that takes measures against electric corrosion

In the unlikely event that electric corrosion occurs, the resistance value of the resistor will change significantly.

In such a case, the failure mode is “open” mode, so it is recommended to use a fail-safe design that ensures safety even when the resistor opens, assuming the worst case.  For example, it is important not to design as in the examples below.

 

(Example 1)

 

Do not use resistors that are easily electrolytically corroded (such as carbon film resistors with the resistance of 100 kΩ or more) for the voltage dividing resistance of series electrolytic capacitors.

In the unlikely event of electric corrosion, the resistors will fail in open mode.

As a result, the shared voltage of the electrolytic capacitor collapses, and overvoltage or reverse voltage may damage the electrolytic capacitors, leading to smoke and ignition.

 

(Example 2)

Do not use resistors that are easily electrolytically corroded (such as carbon film resistors with resistance of 100 kΩ or more) as discharge resistors of power supply circuits.

In the unlikely event of electric corrosion, the resistors will fail in open mode.

As a result, when the power is turned off, the electricity charged in the capacitor may not be discharged.

If anyone touches the outlet plug in that state, the person will get an electric shock.

 

There are many other circuits that can cause problems with open mode failures.

In such circuits, it is important not to use resistors that are prone to electric corrosion as much as possible.

If you have any problems, please feel free to contact us.

 

Related columns:

           What is electric corrosion? – Our efforts