Is Singapore Rain Acidic?What it means for car paint
Yes, mildly: studies measured an average pH of 4.2 to 4.5. The damage starts when a raindrop dries on hot paint.
What the measurements say
Acidity is measured on the pH scale, where 7 is neutral and each step down means ten times more acid. Even clean rain is slightly acidic: carbon dioxide dissolves into it as it falls, which brings it to about pH 5.6, according to the US Environmental Protection Agency.
The most detailed measurements in Singapore come from researchers at the National University of Singapore. Between November 1999 and October 2000, they found the rain was typically acidic, with a mean pH of 4.2. Sulfate was the most abundant ion, and the pollutants came both from long-range sources and from local industry and traffic.
An earlier series of 169 daily samples, collected from August 1997 to July 1998, gave a volume-weighted mean of pH 4.5, with about 88 percent of samples below pH 5. In 2009, the National Environment Agency gave a broader figure of about pH 5, comparable with other cities and no more acidic than in the 1990s.
The academic series are more than twenty years old, and the agency's figure is a rounded one. Together they point the same way: rain in Singapore is acidic, but mildly so.
Why a drying raindrop does the damage
A raindrop that runs off the car does little harm. The problem is the drop that stays. As the water evaporates, the sulfates, nitrates and dust it carried stay behind. The same amount of acid ends up in less and less water, so the last film of liquid on the paint is far more concentrated than the rain that fell.
Heat speeds the process up. The Meteorological Service Singapore records rain on 171 days of an average year, over the 1991–2020 period, and the sun often returns within hours. In the outdoor tests used by the car industry, painted panels left in summer sun in Florida reached about 72 °C.
What acid etching looks like
Carmakers take the problem seriously. The ASTM D7356 laboratory test was written to reproduce the acid rain damage seen on panels exposed from May to mid-August in Jacksonville, Florida, a hot and humid test site. The defects it describes range from small pits to marks about 12.7 mm across, and the standard notes that acid etch is an important warranty claim item for car manufacturers. Singapore has those conditions all year: heat, humidity and frequent rain.
Water spot or etch mark?
The two are often confused. A water spot is a mineral deposit sitting on top of the paint. It comes off with a wash, or with a chemical decontamination if it has been there for a while. An etch mark is a shallow crater in the clear coat itself, and washing does nothing to it. It shows as a dull ring under a torch or in direct sun, even on a clean car.
A light etch that stays within the clear coat can usually be levelled by machine polishing, which removes a few microns around the mark. How car polishing works explains how much clear coat that takes and where the limits are. An etch that goes through the clear coat can only be repainted.
Limiting the effect
- Covered parking. A sheltered lot keeps most of the rain, and the sun that follows, off the paint.
- A rinse before it bakes. Clean water after a heavy shower, before the sun comes back, dilutes what the rain left behind.
- pH-neutral washing. Strong alkaline cleaners strip protection and leave the clear coat exposed sooner.
- A sacrificial layer. Wax, sealant or a coating sits on top of the clear coat and takes the etching first.
Of the three layers, a coating lasts the longest. It is not immune to acid, but once it has worn it can be topped up or renewed, which a clear coat cannot. How a ceramic coating bonds and how long it lasts covers the chemistry, and maintaining a coated car the routine that keeps it working.
Acidic rain works slowly. What decides the damage is how long the residue stays on hot paint.
Sources
- Hu G.P., Balasubramanian R., Wu C.D. (2003). Chemical characterization of rainwater at Singapore. Chemosphere, 51(8), 747–755. PubMed
- Balasubramanian R., Victor T., Chun N. (2001). Chemical and statistical analysis of precipitation in Singapore. Water, Air, and Soil Pollution, 130, 451–456.
- National Environment Agency, quoted in The Straits Times, 14 September 2009.
- US Environmental Protection Agency. What is acid rain?
- Meteorological Service Singapore. Climate normals, 1991–2020.
- ASTM D7356. Accelerated acid etch test for automotive clearcoats.
- Q-Lab. Accelerated acid etch for automotive coatings (LX-5025).
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