Additional Technical Information
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COMPARISON OF MARINE GRADE MATERIALS WITH STANDARD ASTM MATERIALS
We’re often asked what makes marine-grade materials such as Aqualoy 19 or Aqualoy 22/HS better than a standard 304 or 316 stainless steel. Standard materials cost less, so why would someone pay more for marine-grade materials?
To understand the differences in the materials, it’s important to consider the environment in which the materials are expected to perform. Aqualoy 19 and 22/HS were created as specific upgrades to their standard counterparts in both strength and corrosion resistance. Table 1 shows the chemical composition of Aqualoy 19, 304, Aqualoy 22/HS, and 316.
As you can see, the chemical compositions of the alloy pairs are different, with Aqualoy 19 having higher Mn, Cr, and N than standard 304. Aqualoy 22/HS also has higher Mn, Cr, N, Ni, and Mo.
These chemical differences serve to improve alloy performance in two ways. First, the higher Mn and Cr increase the solubility of nitrogen, which is important as nitrogen helps the alloys attain higher strength. Secondly, the higher Cr, Mo, and N help to increase the pitting resistance equivalent number, which is calculated as follows: PREN = %Cr + 3.3 x %Mo + 16 x %N
Two of the most common forms of corrosion of stainless steel in marine environments are pitting and crevice corrosion. Typically, alloys that are more resistant to one mechanism are more resistant to both. This equation has been used for many years to rank austenitic stainless steels and their relative resistance to pitting in chloride type environments such as sea water. Studies have shown that higher PRENs correspond to increased resistance to pitting. If we are to calculate the PREN of the four alloys above, one can easily see the superiority of the marine grades.
grades and calculated prenups
- 304: Calculated PREN 19.14
- Aqualoy 19: Calculated PREN 22.7
- 316: Calculated PREN 24.07
- Aqualoy 22/HS: Calculated PREN 34.06
Annealed Yield Strength (psi)
- 304: 30,000
- Aqualoy 19: 50,000
- 316: 30,000
- Aqualoy 22/HS: 55,000
From the chemistry modifications, you’ll see improved corrosion resistance with the marine grades in marine environments.
As far as mechanical properties are concerned, the differences are also dramatic. In fact, the original design criteria for Aqualoy 22/HS were to double the strength of 316 at the same time as the corrosion resistance was improved. As mentioned earlier, the primary modifications to the chemistry were aimed at increased nitrogen content—and when you look at the annealed yield strength of the four alloys, the improvements are obvious.
In addition to the chemical modifications of the marine grades, there are processing modifications that are designed to increase the strength dramatically above the annealed properties listed above. Depending on the size, it’s possible to increase the yield strength of both Aqualoy 19 and Aqualoy 22/HS in excess of 105,000 psi. This will allow more power to be transferred through marine grades of the same size when compared to their standard counterparts.
DOWNLOAD OUR COMPREHENSIVE DATA BULLETIN
Includes:
- Chemical Compositions
- Mechanical and Physical Properties
- Shaft Tolerances
- Reference Tables
- Shaft and Bearing Selection
- Machining and Installation
- Marine Shafting Repair
SHAFT DIAMETER AND SAFETY FACTOR CALCULATOR
A Safety Factor–Shaft Sizing–Bearing Spacing Program* is also available. WBM offers this simple program to enable customers to calculate safety factors and shaft diameters on their own computers. The program will operate on any version of Windows.
Where:
- D is the shaft diameter
- P is the rated horsepower
- S.F. is the safety factor chosen
- S is the yield strength in torsion
- N is the shaft rpm
321,000 is a factor derived from converting horsepower to torque/balancing units.
Note that the yield strength in torsion is a calculated value from the tensile yield strength according to ABYC-P-6.
*Note: Shaft sizing calculation can be found in ABYC-P-6.
cathodic protection
Aqualoy shafting materials are designed to offer the best combination of strength and corrosion resistance for their respective alloy groups. In order to ensure a long shaft life, free from corrosion, we recommend using sacrificial anodes for all grades. Our other recommendations include:
- Boats that are only used in salt and brackish water: Use Navy grade zinc anodes.
- Boats used primarily in fresh water: Use magnesium or aluminum anodes.
- Boats used in fresh brackish and salt water: Use aluminum anodes.
Please note that in fresh water, zinc anodes will become ineffective in a very short period of time, leaving all submerged metallic parts unprotected.
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