The guidance states that 304L may be used at chloride levels of about 200 ppm, 316/316L at levels up to about 1,000 ppm, and duplex 2205 at levels up to about 3,600 ppm [S6].
The comparison applies to austenitic stainless piping and equipment. The cited water-service guidance identifies free-chlorine limits of 2 ppm for 304 and 304L and 4 ppm for 316 and 316L [S2][S8].
Recommended Chloride Selection Bands
These bands are more defensible for routine industrial-water selection than treating either austenitic grade as universally chloride-proof.
Underground-service guidance gives a similar distinction: 304L may be used at chloride levels of 200 ppm, 316/316L at about 1,000 ppm, and duplex 2205 at about 3,600 ppm [S6].
Use the lower result when assessing a system. A nominal supply-water value below 200 ppm does not override stagnant downstream zones, evaporated residues, gasket gaps, deposits, or intermittent seawater intrusion. Pitting and crevice corrosion depend on temperature and geometry as well as bulk chloride concentration [S4][S5].
Why 316L Extends the Chloride Range
Type 316L contains approximately 16–18% chromium, 10–14% nickel and 2–3% molybdenum, while 304 contains approximately 18% chromium and 8% nickel without molybdenum [S1][S3]. Molybdenum makes the passive layer of 316L more resistant to attack by chlorides than that of 304L [S3].
The “L” denotes low carbon, and the cited analysis identifies weldability as the reason for using the low-carbon grade [S5]. It does not remove the need to inspect passive surfaces, remove weld heat tint, drain after hydrotesting, or protect creviced components [S4].
The guidance treats chloride limits as conditional on water chemistry, temperature, and localized corrosion mechanisms, and states that 304 and 304L have been used successfully in fresh waters containing up to 100 ppm chloride, particularly where crevices are present [S2].
Temperature, Crevices and Free Chlorine
Chloride concentration alone does not define the corrosion threshold. These values apply to the cited cooling-water context and show why ambient-water tables cannot be transferred directly to hot equipment.
Both 316 and Type 304, including their low-carbon variants, are susceptible under certain conditions to pitting, stress corrosion cracking, and crevice corrosion in cooling water containing chlorides [S5]. Residual weld stress, cold-work stress and pitting can increase cracking risk, particularly in heat-exchanger tubing with 16 BWG, 0.065-inch walls or 18 BWG, 0.049-inch walls [S5].
Free chlorine is a separate limit because it is a strong oxidizer that can accelerate localized attack. The cited guidance permits 2 ppm chlorine for 304, 304L and 321, versus 4 ppm for 316 and 316L [S2][S8]. Equipment warranties and tested operating envelopes can therefore be tighter than general alloy-selection guidance.
304L Versus 316L Decision Matrix
For bulk chloride below 200 ppm, 304L can remain defensible when temperature is ambient, crevices are controlled, chlorine is at or below 2 ppm, and the water chemistry is stable [S2][S4]. 316L contains roughly 2–3% molybdenum, which 304L lacks, and molybdenum makes the passive layer more resistant to attack by chlorides, while 316L costs more than 304L [S3].
At 200–1,000 ppm chloride, select 316L as the minimum austenitic grade for controlled industrial-water service [S4][S6]. It provides materially better pitting and crevice resistance through its 2–3% molybdenum content [S1][S3], but it remains vulnerable to chloride SCC at elevated temperature and does not justify relaxed control of crevices, deposits, residual weld stress or free chlorine [S5].
A published comparison states that SS304 can begin to show pitting corrosion in environments above roughly 150–200 ppm chloride, especially at elevated temperatures [S7]. At water temperatures above about 60°C, use laboratory test data or a more highly alloyed grade when crevice geometry and concentration effects are material.
A voided chloride specification should distinguish chloride from residual chlorine. Chloride is reported as Cl⁻ concentration, while free chlorine is separately controlled as Cl₂-equivalent exposure in the cited guidance [S2]. If either value can drift, specify 316L only after establishing maximum chloride, chlorine dose, temperature, pH, flow, deposit chemistry and crevice details.
Water-Service Applications and Exclusions
Type 304L remains suitable for many domestic water-storage applications, treated solar-thermal fill water, dry service and selected low-chloride potable-water systems when the chloride limit remains below 100–200 ppm [S2][S3]. Type 316L is the better engineering choice for chlorinated water, coastal exposure, brines, salt solutions, water-softener regeneration service, hot water and cleaning chemicals containing chlorides [S3].
Neither grade is a general substitute for duplex stainless steel in higher-chloride service. The supplied selection bands move from 316L to duplex 2205 at approximately 1,000–3,600 ppm chloride, with 6% Mo super-austenitic and super-duplex grades reserved for still higher concentrations [S4]. For seawater exposure, the cited guidance also flags the limited SCC margin of 304 and 316 [S5].
For potable systems, verify applicable drinking-water approvals separately from corrosion suitability. A material selected because it tolerates 1,000 ppm chloride may still need approval for the pipe, fittings, sealants, joining method and complete assembly. The corrosion bands in the cited documents do not constitute a local code or drinking-water approval.
Commissioning and Lifetime Controls
Specify the maximum values rather than nominal feed-water values: 304L should be checked against 200 ppm chloride and 2 ppm free chlorine, while 316L should be checked against 1,000 ppm chloride and 4 ppm free chlorine for routine water-service guidance [S2][S4][S8]. These are screening ceilings, not guaranteed corrosion-free conditions.
Before hydrotest, remove heat tint, drain the system promptly and avoid trapped chlorinated water; these controls appear in the wastewater-treatment selection guidance [S4]. Monitor free chlorine, chloride, temperature and pH, inspect welds, gaskets, tube sheets, heat exchangers and dead legs, and remove scale or deposits that can concentrate chloride beneath the surface [S5].
For long-life or difficult-to-replace equipment, use the highest credible future water chemistry when selecting the grade. A supplier warranty boundary such as 1.5 ppm chlorine for T-304 or 8 ppm for T-316 is not a general corrosion threshold [S3]. The next engineering decision is a documented envelope containing maximum chloride, maximum free chlorine, operating temperature, pH range, crevice geometry and expected service life.